Many sellers don't want to do this.

Consider someone who inherits their parents house, 1000 miles away. The house needs renovation.

You want them to uproot their lives or find a "service provider" (1000 miles away) instead of just doing an "as is" sale to someone who will do the renovation and sell it.

Sellers want out. There's no social benefit to making that more difficult.

Many buyers aren't interested in doing a renovation, and most of the other things apply.

People who renovate for a living do have those things.

You seem to think that holding out for more money is no-cost - you're wrong.

Flippers lose money if they're wrong about the market, so why are you so insistent that it's wrong for them to make money?

Which reminds me, how long does someone have to own something for you to be okay with them selling?

People can sell whenever they want, my restriction is the profit margin. I have no problem with that margin being more and more over time so long as that rate is nationally set to be uniform.

Flipping as a service requires waiting by buyers and/or sellers which costs them money. Why is that a good thing? (A renovation may take 6-12 months.)

That's a general contractor, not a flipper. The key difference is who takes on the risk: if I hire a general contractor and they turn out to be not very good at their job, that's my problem.