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.