Why does Apple insist on using .mov files?

.mov files are consistently 10x larger than .mp4 files for seemingly no difference in video or audio quality.

That would be because the default you're using is uncompressed video. So yes, there's a difference. The video hasn't had data averaged out yet. Depending on the app you're using, you should be able to choose a compression scheme other than Uncompressed when creating the original .mov footage.

You also don't need to convert anything. Just change the extension from .mov to .mp4.

What do you mean by "mov" file? It sounds like you are complaining about some specific mov file. Or perhaps the files generated by a specific app. If so, it would be better to address that question in a forum dedicated to that app.

Generally speaking, a "mov" file is a QuickTime video container. Its size is governed by the video contained within. If your mov files are too large, then that is a question for the app that is creating them. The most logical explanation is that some app is creating a mov file that contains several different versions of the video for faster export to a variety of different formats. Otherwise, you would have to re-encode each time you exported. Apple's priorities are user experience, performance, and energy - most definitely not storage. So it makes sense Apple would develop a system that provides fast exports, with minimum CPU usage, at a cost of storage.