Early iPhones actually did allow custom ringtones, but required converting MP3s to AAC/M4R format and syncing through iTunes with specific file length restrictions (30 seconds max). This technical friction compared to drag-and-drop MP3 transfers on other phones likely accelerated the ringtone market's decline.

You can still make custom ring tones this way if you have access to a Mac laptop. You make a m4a file, rename it to m4r, and transfer it to the phone using finder.

I remember when cellphones first allowed on-phone media, and companies like verizon prevented you from putting your own media on your phone.

and apple saved people from that...

but then at some point, apple music came along and deprecated local music. (enough that many people were confused and switched to apple cloud music)

Android removed it from the UI not long afterwards, though they never actually removed the functionality - any mp3 would show up as if it was a built-in ringtone if you created a directory with the right name on your sdcard and put the files in there.

I just went into the sound settings on my two-year-old Pixel, and under “Phone ringtone” there was a selector that was perfectly willing to pop up a file picker and accept a random MP3 file I had lying around. It’s more friction than I think is reasonable for a non-power-user feature, but if you expect the function to be there, you’ll find it by choosing the obvious-looking option at each step.

Apple ring tones were a bit of a status symbol at the time so people mostly kept them