The same reason I haven’t stuck with HSBC, or don’t go into a physical branch every time I want to carry out banking transactions. I understand there are other ways of doing things, but I’m trying to suggest ways which I think the Monzo app could work better for me, in the way I want to work and hopefully, other people will find it useful too.
Personally, I have a couple of recurring payments which I’d love to schedule, but they aren’t consistent enough that I feel comfortable setting up a standing order and it mildly stresses me out every week because either it’s on my mind more than it should, or I forget about it and hence forget to pay. Having some functionality in the app to assist with these things would instantly mean that I don’t have to remember to pay or worry about forgetting. It could be useful for other things I didn’t originally think of, as @Throwingspoon has already pointed out.