Not to speak for the actual engineers of the platform... But, you're asking for a very specific thing.
As far as I understand it, it is much easier for them to upload a standard streaming format - which works for the majority.... And offer a downloadable version... If neither of those happens to coincide with the format that is convenient to you, there are innumerable options of programs or applications that can reformat the downloaded copies into the proper file types...
Hey, I wouldn't mind an easy, condensed, mp3 format for download.... But - if you're not up to date with the standard tech to play the downloaded files - it's not necessarily on them...
You paid for a ton more content... It's not their fault you only wanted one thing - and it's not on them to reformat every recording to make sure it works for your specific needs... It's actually pretty rare that someone needs an ONLY- mp3 format, nowadays...