It's much of the same reason if you turn on your TV and watch any NASCAR or Formula 1 event you notice how boring it is...all the damn cars look the same! It's so lame!!
I said that tongue in cheek but that's really a lot of it.
There are 2 schools of thought from the perspective of a site owner:
1) Do what I like and assume my customers are like me
2) Do what works and what my customers want, even if I don't like it so much
I don't mind divulging that the only reason we started adding multiple design themes and will be adding more wasn't to help pay sites make more sales but because enough pay site owners feel the same way you do. It doesn't matter if I point to sites using an out of the box Elevated X setup (or one with very little customization) that's 100x bigger and more successful financially than most pay site owners even hope to be. Bottom line is most people want what they want.
We've had plenty of clients customize their sites to the degree that they are literally unrecognizable as Elevated X, even to us.
We've also had clients cancel service to build a custom CMS so they can have things just how they want in all areas.
Even if they hate his personality, most can agree that few people are better at what he does than Donald Trump and one of his trust statements is that you can't successfully sell something you don't believe in.
If you want total control and want something unique my suggestion is to go custom and spend a year or so migrating your better selling or higher potential sites by hand if necessary.
You've got some good stuff going there and I think regardless of "how" you choose to do it you'll benefit long term from modernization.
Feel free to email or skype me anytime (with no obligation) if you want to bounce ideas around or weigh the pros and cons of whatever you're considering doing.
AJ