Also, you're making second bad assumption - that because it takes you a long time to adapt stuff compared to generating it, that's fairly universal. I do not believe that's true. I mean, take me or my wife - we can certainly write our own adventure faster than we can prep a pre-made one (you sound the same way). My brother, though, or most DMs I know? Definitely the other way - and indeed many of them don't even significantly modify the adventure for their group. You're welcome to sneer at that, but it's how they roll.
Maybe. But if I have to go through the stat blocks and figure out what each spell and feat does (because why put that information right there where I need it) and I have to draw plot flow charts and NPC relationships webs and all the rest, I might as well write the damn thing myself. Most adventures are terribly produced from a utilitarian standpoint. They don't give you the information you need where and when you need it. Whether this is a huge problem depends on your point of view, of course, but for me it basically means no matter how "good" a module or AP is, it is not worth the effort -- and players (especially those at cons orgame days) are none the wiser. Everybody has a good time, including me.