This is a question for support but most likely has to do with Apples strict rules about who is allowed to and how they are allowed to distribute apps. It’s also easier for pretty much anybody to get the apps on their phones this way. Not everyone has a mac computer but may have an iphone. This would make it impossible to do anything with their IPA.
However, you can upload your android apps to your android phone with either a mac/windows/linux setup. It’s a pain but is actually a very nice feature.
If you want the .ipa, you could download iMazing. I believe it is a 1-time 40$ fee. This will allow you to rip .IPA’s of any app on your iphone, and allows you to load it onto any other iPhone. All without jailbreaking (i think)