Mariah is an actual writer and producer, not a phony one, so naturally she's more invested in the songs and the process takes longer.
It's easy for these singers who just come in, sing the song and leave to record loads of tracks that are given to them and then choose which they are going to use.
Beyonce saying she records that many songs for an album just confirms what we already know about her 'writing'.
That's definitely the reason Mariah won't have a huge collection of unreleased tracks sitting in a vault somewhere.