> I'm surprised an enterprising company hasn't come along that allows you to pick all your own stocks, and allow them to be rolled up in an index. That kind of service shouldn't be too hard to create from a technical point of view, so I'm assuming the issue is with regulation.

This is what Motif does, but it's super expensive, and picking stocks is a terrible idea. Vanguard/Betterment are cheap because everyone owns the same thing and you can trade between customers for free.

https://www.motifinvesting.com