Why do prestige classes have prerequisites?

[Dr Simon]

Why do prestige classes have prerequisites in terms of feats, skills, BAB, etc?

Probably because they are inspired by the Rune Lord and Rune Priest "classes" of RuneQuest, where you needed certain scores in certain skills (or to know certain spells, or to have a certain amount of magic etc.) to qualify. Of course, in RQ, membership in certain organisations was implicit, if not downright explicit. You could never, for example, hope to become a Sword of Humakt if you weren't already an Initiate of Humakt. It is easier to do this in RQ since there are no such thing as "class skills", thus you can work towards specific skill requirements without performing the multi-class quick-step.

I think Pr-classes in general have become rather watered-down and tend to veer into "kewl powerz" territory, where the intent was for them to be specific branches of specific organisations (or at the very least, ways of thought and/or training).

Anyone can be an assassin de facto by making a living killing people, but it ought to be (for example) that only someone who has joined the Slayers Brotherhood learns their secret magic, trains in poison use and is taught the powers of the Death Attack. Insert your own campaign-specific black-clad organisation here.

Some of the newer Pr-classes in the 3.5 DMG seem more to me, well, here's a sort of multi-class thing without the penalties. A bit dull. That's when they become more a question of meeting numerical requirements than any RP requirements.

Side rant: I agree about the ridiculous gymnastics that are done to avoid *specifically* excluding classes, but effectively to do so. The Eldeen Ranger from Eberron requires "Favoured Enemy class feature". Welll, that'll be the ranger then. Why not say that?

 

[Flyspeck23]

Side rant: I agree about the ridiculous gymnastics that are done to avoid *specifically* excluding classes, but effectively to do so. The Eldeen Ranger from Eberron requires "Favoured Enemy class feature". Welll, that'll be the ranger then. Why not say that?

Because there might be other classes that grant that class ability. Another prestige class perhaps?
It makes perfect sense that most Eldeen Rangers will be, well, rangers. But simply saying "Requirement: 1 ranger level" would be bad style (and not necessary).