Who says? People who watched baseball in the 60's and 70's (when you couldn't watch nearly as much as you can today) like to think that pitcher's were men and batters were sheep back then. I don't buy it. BTW, I've been watching (and paying attention) since the late 60's/early 70's and I don't think what you say is really true. I think there's a bit of a myth that some announcers and some fans just like to continue (there's a better word that you probably know).
Well, since you've been watching baseball since then (and I fully believe you have), then you know that it was completely routine for P's then to "back guys off the plate"... which doesn't mean they hit them, but it does mean they threw to locations that are now considered provocations that aren't tolerated...
Plus, with guys who lean over the plate wearing armor now, hitting them may or may not occur more, but when it does happen, people get upset, umps issue warnings, etc. I'm not saying guys got hit more back then because I have no idea if they were or not... but I am saying throwing inside to "own the plate" used to be completely normal but is now something that's simply not permitted as it once was...
Just think about that for a sec... you know it's true...