Why do you only learn a new form after your next gup?

So I've been study martial arts for a year now and I still don't get why you have to wait to learn a new form after you grade since I usually get the new form down in about a month to a month and a half which means I'm busy waiting to get graded. I mean I still have two Bo-Staff forms to learn but I honestly view the bo-staff as kind of boring despite how vastile it is since anything can basically be a bo-staff. So I've been thinking should I attempt to create my own forms since I know it's one of the requirements to get your black belt. I'm sorry if this bothers anyone and it actually makes me feel bad I feel this way since the last few times I've gone to the dojo I felt like I could just be training in my backyard until the next grading...