It’s because writing software costs money. Especially when you have actual customers who need support and can’t say “do a GitHub fork and fix it yourself.”
As an user, sure, side-loading ePub directly would be nice.
As a developer I’d politely inform my project manager that this would require multiple months and that she has to tell her superior that we either need more persons or that the x-ray feature will have to be delayed or that this could severely impact battery time or whatever possible implications are.
The kindle exists to sell books from Amazon in an as convienient way as possible, so that an octogenarian can use it. With as little help from a hotline as possible, preferably none, because those also cost money and when a user has to call it, it’s very likely because there was a mistake that shouldn’t have happened.
There are literally dozens of alternatives with another business model, where you just get the hardware.
