Probably a combined problem
Ignoring the obvious possibility that they use the man-in-the-middle server to exploit the data of their users, there are other ways to explain this.
Modern app developers, and you can assume that most of them are new to programming, go by the design pattern they are used to. And those include using an external server accessed via HTTP/Websockets, instead of doing local computation. They have been taught that local computation is slow and battery draining, so they do remote computation which requires data communications... which is slow and battery draining. Nobody does Profiling to see which way would be better in that situation. Furthermore they have never been taught in the ethical aspects of their trade, so they don't understand why it's a bad idea to more external components than necessary.
Then some mobile operating systems don't support "raw" sockets so you could do IMAPs. Windows Phone, for example, didn't support it on early versions. Plus there may be a certain irrational believe that using raw sockets is somehow bad, and you should have a layer in between.
Now if you actually control that server in the middle, the concept may actually even make sense. Done right, you can avoid having to store e-mail on your mobile device, which means that it'll be secure against theft. A server is much easier to secure than a mobile device since you can literally guard it from physical access by your attackers, and you can reach a far higher level of FOSS on your server.