There are some people who I call 'kit people'. They don't want to build up a stash, for whatever reason. They only want enough supplies to make the project at hand and they don't want any leftovers.
There are people who may not be as fortunate as you and have other constraints that prevent them from devoting all their free time or money to the craft. If you only buy BOMs, it's a lot cheaper than going out and buying fabric that just sits in a box or on a shelf until you decide it's time to use it a quilt (if that time ever comes). I still have yardage I bought 15 years ago.
By doing a BOM, it also takes a large task and divides into smaller tasks and that gives the person a sense of accomplishment. They are no longer looking at this 'huge' quilt they have to complete.
BOMs are also good for beginners. I was fortunate that when I learned to quilt, a class was given at the local community college through their community outreach program. Many communities don't have that opportunity. Most quilt shops don't offer a beginner's class every week.
BOMs may not be for everyone but they do have their place in the quilting world.