I think part of it is regular inertia, but part of it is the RIAA protecting their monopoly. It's hard to get your CD in stores as an independent, it's hard to get your songs on the radio or your video on MTV. And it's not all because you're unknown, there's all sorts of things like payola which come into play.
Yes, record companies do provide marketing muscle, but not equally. It's something of a gamble, because although it may be nearly impossible to get radio play without a record contract, signing the contract doesn't guarantee you radio play. Once the company has your signature, they own your work. They will only spend money promoting it to the extent that they think they will get a return. And once you sign, you've lost the right to explore other avenues, even if the company isn't promoting you.