There are a few situations where getting accepted into YC is really the only chance a founding team has. A combination of young kids with financial hardship and being new to the tech market. They have no connections to work, no contracting jobs to support themselves with, and taking a full-time job is a 1-2 two year commitment.

Other seed funding providers are popping up, sure but if you're short on money you might not have time to wait for their next deadline.