Because the API is a REST API and you don't want your frontend to have to make 100 calls to render a screen. Each frontend may be a completely different app so they all need the data in their own format. And each team may control their app so you don't want all of the teams roadblocked by the API team.
If the API were a REST API then the client would only need to make 1 call to request the screen (representation of a resource) in a format (media type) the client expects.
This is not the reality when you go into most organizations. It may have started like that but a few cases of "just one more call" result in the client making a load of calls at startup followed by many others when user navigates to a detail screen. It probably caches a few items, but many others are not cached appropriately.
This is what the vast majority of developers are facing. Be thankful, if you don't have to clean these messes up only to move to another team to repeat it. It happens naturally because of the tension between shipping things and finding enough time to properly optimize.
