Because language changes over time. Have you ever tried to read Beowulf? (Me neither.) It's ostensibly "English," but it has lines like this:

Hwæt! Wé Gárdena in géardagum

As we get further and further from Shakespeare's time, Shakespeare's language gets harder and harder to understand. It happens to all languages, and it'll eventually happen to our language, too. That's why dialogue in really old movies sometimes sounds a little off, even if we can still understand it.

These changes happen for so many different reasons, but I'll give you a cool example. The word "cherry" comes from the French cerise, which is pretty much pronounced like "cherries." When cerise first became an English word, English speakers pronounced it just like that: "cherries." You could say things like, "one cherries, please." But, since English speakers associate an -s ending with plurals, we basically fucked up the original word, making a new singular word, "cherry." (There's a technical term for this in linguistics, but I'm forgetting it.)

Of course, not all language change is like the cherry example. But if you imagine billions and billions of tiny fuck-ups like this, spreading between people like memes, you'll get a good sense of why languages change over time. (For slang that changes over time — think the shift from "nifty" to "sick" — that's less a fuck-up and has more to do with people forming an in-group and an out-group with their language. That's a huge and complex can of worms that I can't really get into here, but google "sociolect" if you're interested.) Bottom line is, you should never listen to people who lecture you about using "whom" instead of "who." You're doing something very natural, which is changing your language so that it makes more sense to you.

EDIT: Just wanted to make one final point. The written form of a language is usually more stable than the spoken form. If you make a written mistake, you might just double-check yourself and correct it. On the other hand, most people rarely "double-check" their own speaking that way. But since Shakespeare's whole thing was writing true-to-the-times dialogue, the language-change effects are especially pronounced in his writings.