Why so many for monkey island 2?

Because it's such a great game? I don't know.

Because I like playing the game, and when I played it for the sake of reviewing, I finished it. And while I was at it, why not document the whole game? Sure, it may be counted as a spoiler, but it's up to every visitor to decide how much he or she wants to see. The offer is there. Oh, and by the way - 166 for Indy 4

A large amount of screenshots is part of documenting a game appropriately in my opinion. It's a reliable criteria to distinguish between quality sites and cheap sites that don't have a dedicated webmaster (who hasn't even played the games he presents).

Personally, I'll never put up many screenshots. When I look for a game, I don't even look at more than three or so, generally. When I make screenshots, I only want to give a small taste of the game, not completely document it. But then again, that's dependent on review styles.

Try to look at it this way: Already, only a tiny minority of all people is able to run these games at all. In the future, this group will shrink even more. So, 'museums' have to fill the gap: Sites which 'document' the games for 'passive' consumption, since 'active' isn't possible anymore.

Good reason, however, I think you're forgetting emulators. There are very few old systems that don't have one.

No, I'm not forgetting emulators, but specifically including them. The number of people who even only know about the existence of emulators is shrinking day by day, let alone the tiny group which is able to handle them.