OEMs and Stock Apps

I'm really curious as to why OEMs do not/can not update individual stock apps, such as email, text messaging, etc.? Like do they not have people working on them or is it not possible to push out those kind of updates to people without updating the entire OS? Like for instance I have a Razr MAXX and I get updates from things that I never use such as MOTOPrint, yet the E-mail app is slow and buggy and will never see an update until I get a full OS upgrade. Why is this?