That´s pretty normal these days on many brands (including Oneplus).
I think, most people either don´t have a music player on their phone they use, or they use spotify or anything similar to spotify. So why bother making one?
I´d prefer no music player over a half assed one I may not be able to uninstall. And there are plenty of GOOD options out there, that aren´t “just for Samsung” or whoever.