Those who receive an MFA often end up teaching. A lot of people who pursue those extra years of study however, just want two years immersed in a community that will teach them, challenge them, and give them space to produce new work.
If someone doesn't want to become a creative writing professor, why do they have to take years of extra education to receive the two years they crave? I understand having a wider knowledge base at the start of grad school helps. I realize using the education that came before can mean all students have a more equal field. But, it still doesn't sufficiently explain it as there can be an application process where potential students prove their skill.
There are Bachelor's Degrees in Creative Writing, but most of them are like every other BA: The kitchen sink approach where giving students a varied curriculum is more important than the writing itself. There isn't, to my knowledge, an undergraduate degree that offers the connection, support, and focus an MFA grants.
Sure, people can piece together resources that mimic the experience. A writing group can be found or forged. Books and blogs on nearly anything taught in a college course can be bought or borrowed. A writer can volunteer at a literary magazine as a "slush reader" and gain valuable insight. But the "real world" will be able to intrude more often, time will be spent cultivating information a professor would provide, the writing group might not be as concrete as a classroom of peers, etc.
Writers (and everyone else) need options so they can choose what feels right for them.
I think about my days at college. If an Associate's in Creative Writing had been an option back then, I could have tried it. The knowledge I gained from the time I was in college hasn't proven useful in my everyday life. I don't even have an Associate's after everything I went through, but at least I would have knowledge to help me as a poet.