Forcing the simplification of all those taxes doesn't seem like it has a downside, to me.

That would centralize power to the larger taxing authority.

Right now, there's a huge number of elected people in the US who wield real local power through these taxes and other rules that they can make.

It's a headache but we live in the computer age and we can automate administrative things like tax calculation at checkout; we should be using systems to aid decentralization and democratization instead of the opposite.