I wonder if these kind of contract arrangments are just a consequence of lack of communication. You have to agree on a single cost because it is impossible to

I wonder if these kind of contract arrangments are just a consequence of lack of communication. You have to agree on a single cost because it is impossible to audit and check everything the contractor does. But the client will inevatably have to check everything anyway but in a delayed fashion. So why not assume from the outset that the client is just another part of the team and must sign-of on everything every single day. Give the client complete access to all data about the project. With sufficient communication the contractor could prove they are working correctly and the client justs pay them for the time and materials they use.