> it would be better if these platforms were neutral entities that just connected sellers to buyers in an efficient way. But they try to totally control the sellers.
This is more true of Apple than of Uber or Amazon. There are ways to build your own reputation. You can operate your own store and your own website and also at the same time make your products/services available via Amazon for the people who look for them there first, and include information about your own website on the retail product packaging. The real problem is that building a reputation is hard and a lot of people will fail. The large majority of small business fail. That isn't something Amazon invented or caused.
By contrast, what Apple is doing would be like there being a city where Uber owns the roads. You can go where you want (unless they decide you can't), but you have to take an Uber, you can't use Lyft or a taxi or buy your own car or get a ride from a friend.
Which is a much bigger problem, because it prevents anyone from going from client to competitor. It prevents, for example, what Valve does with Steam on Windows (and other platforms), or Amazon or F-Droid does on Android. Even once you're a big enough producer to justify doing your own distribution, you still can't. But that's how distribution competitors come about, which means there can't be any, and then you're completely at the mercy of the monopoly distributor. If they decide they don't like you, or they start to compete with you in your market, or they just fat finger some paperwork, you're completely excluded from the market. Can't switch to Lyft or Walmart, can't strike out on your own, you're just dead in the water.
Most companies will never make it to the point where that matters, but the ones who do are really important because that's where competition in distribution comes from. Recall that Amazon started off as a book store at a time when Walmart was considered unstoppable.
