I dunno about Photoshop - I never use it - but in Illustrator, a lot of core program functionality is implemented as plugins nowadays. For CS5.1, plug-ins/extensions has 53 items, plug-ins/tools has 53.

I guess they could do some kind of lazy loading, but that'd be, you know, work to implement, and they're too busy with stuff like their own (broken) custom window routines to get Windowsesque MDI on the Mac.