Since the start of the 20th century, only a select few baseball players have batted at least .400 for a season:

1901 .426 Nap Lajoie
1911 .420 Ty Cobb
1911 .408 Joe Jackson
1912 .409 Ty Cobb
1920 .407 George Sisler
1922 .401 Rogers Hornsby
1922 .420 George Sisler
1922 .401 Ty Cobb
1923 .403 Harry Heilmann
1924 .424 Rogers Hornsby
1925 .403 Rogers Hornsby
1930 .401 Bill Terry
1941 .406 Ted Williams

Are players not hitting as well anymore? Are they hitting for home runs rather than base hits and thus striking out more? Or, as Stephen Jay Gould has theorized, is baseball as a system simply reaching equilibrium, in that there are also fewer very bad seasons? Remember, the information and training put into a ballplayer nowadays would have been unthinkable in the 1920s. Batters and pitchers both know what they're facing before the anthem plays- Perhaps they've just reached a balance.

In more recent history, George Brett finished his 1980 season with a .390 average, hitting .408 as late as August 28, and Todd Helton peaked at .421 on May 31, 2000, to end that season with .372.

(The following is an editorial on the .400 average):

My father and I used to discuss the elusive goal of batting .400 (both of us are baseball fans, but not rabidly so). We came to the likely conclusion that the demise of the .400 average is largely due to a concept we take for granted today: Relief pitching.

Baseball has scaled well in talent since the turn of the century. The scores have been about the same... the averages have been about the same... the players have been a good match for each other (for the most part). Training has gotten more intense for both pitching and batting. Both of their arts have improved and matured technically until they have reached where they are today. Why has relief pitching helped so much? Because in no place in baseball do you see an area where stamina is so important (and having a large bench on your team so beneficial) than in pitching.

It is easy to notice that pitching takes more long term effort than batting (and other positions in fielding). Rarely these days do you see a person who pitches nine innings and lives to tell the tale (if you will). Many times you will see people who get put in for only one batter or two. There are usually two types of pitchers: starters and closers. Roughly, the starters have the power and the endurance, while the closers have the short burst of fire to retire the sides.

Back then, a fairly fresh batter could destroy the pitches of a very tired hucker in the eight inning. Slow pitches to the strike zone make for an easy target for the home run hungry. We immortalize those baseball greats that weren't athletes (look at Babe Ruth), but rather heroes. Those who made the hit in the pinch... the everyday man who could pick up his plank and destroy a ball in the bottom of the ninth...That is what made those players legends, not the stats.

Those are the Hall-of-Famers in the .400 list: the CrackerJack boys; the baseball cards with a stick of gum idols. Those were the days before pitches were marked with speed guns, before there was stats on what they do while they are in the bullpen. These people played in a time when baseball was a game, and not a sport.

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In response to OJ, this also happened to Ted Williams, the last of the 400-club:
The intentional walk: As my grandmother often recalls, Ted would step right over and swing at them. Since he was so likely to get a hit (hitting over .400 by any measure is excellent), oftentimes in a pinch, pitchers and coaches would call for the intentional walk. While not particularly fair (but massively strategic), this tactic slowly hurt William's average.

I think there are several reasons we haven't seen a major leaguer hit .400 since 1941. Part of it is because of the rise of relief pitching, although I would argue that it has less to do with stamina and more to do with familiarity, scouting, and platoon differential.

If you face a pitcher three or four times a game, multiple times throughout the season, you will see his entire repetoire of pitches. Whether he's throwing smoke or not, you're better equipped to deal with whatever he throws. That doesn't happen anymore. There are more teams in each league, so hitters faces each pitcher fewer times. Starters pitch every fifth day, instead of every fourth, so hitters faces each pitcher fewer times. And starters don't pitch as deep into ballgames, so hitter face each pitcher fewer times. Add all of that up, and as a batter you're less and less familiar with pitching as time goes by.

Some other reasons there hasn't been a .400 hitter.

The increased importance of the home run. Fans want home runs... they love home runs. It's great if you can hit 200 singles in a season and play rock-solid defense. If you don't hit home runs, people don't come out to see you. Swinging for the fences results in a lot of fly ball outs. And since you're swinging for the fences, you're not sacrifice bunting, which is a great way to NOT hurt your average.

The decreased importance of average. It has become less and less important to hit for average. Statistics like on-base percentage and slugging percentage are considered a better benchmark of hitting prowess.

More night games. With the exception of the Cubs, there are almost no day games outside of the weekends. It's much easier to hit during the day than at night. Todd Helton batted 29 points higher during the day than at night. Nomar Garciaparra hit 18 points higher.

Increased travel time. Prior to 1950, the furthest south AND west a team would travel was St. Louis. Three teams in New York, two in Boston, two in Philly, two in Chicago. Not a lot of travel time. No late flights. No West Coast road trips.

The media hounds. Any player hitting .400 or better in June can expect daily interview requests from reporters. June! Unless you're really good at dealing with the media (read: Mark McGwire), the pressure of a constant media circus is too much to handle.

There are certainly some other reasons why no one has hit .400, but maybe the best reason is this: It's fucking tough. Hitting a baseball is the hardest thing in sports to do. And if you only fail 70% of the time, you're considered one of the greats.