In general, though, it goes like this. Back in, say, 1910, there were a bunch of players who were simply better players than league average and could routinely hit .350 or better. The reasons why varied--perhaps they were better athletes, perhaps they had better vision, perhaps they remember specific pitchers and their tendencies better, perhaps they were better contact hitters, or perhaps a million other things. Over the last 100 years, though, many of those differences among players have been ameliorated, and some have completely disappeared. For example, there are probably zero baseball players today with worse than 20-20 vision, but that might not have been the case in 1910. Also, all teams and players have access to volumes and volumes (or hard drives and hard drives) full of data on pitchers' past performance, whereas back in 1910 a lot of it probably relied in good memory, written records, and largely on word-of-mouth. It was probably routine in those days for a batter to face a pitcher he had heard a lot about but had never seen. These days, no batter (even a fresh minor leaguer) ever faces a pitcher he hasn't at least seen on video.

Overall, today I think we see a much more even field of players, for both the hitters and the pitchers, and the difference between the best major leaguer and the worst major leaguer is much smaller than it used to be.

My dad'll say something like, "Well no one hits .385 anymore!" and I try to tell him that the league is just very normalized nowadays - for an array of reasons, there is less difference between the best and worst players in the league, and statistically there will be very rare instances of an extreme type season (.390 BA, .500 OBP, .800 SLG - though that has always been rare because it's nearly impossible), because the competition is so stiff across the board. Does that make sense?