The vast majority of (moderately successful) startups I see friends and colleagues starting or working for are in this group. For every one friend I see go work for a company like Uber, I see 10 more working for a startup that builds software for expense reports or HR teams or insurance companies or old school cab companies. They will never be billion dollar companies, and they probably won't IPO, and they may be an acquisition target -- but that certainly isn't their end goal. They are gaining customers and growing relatively sustainably, making smart choices about when to (or not) take outside funding.
It's in their request for startups though. Seed funds don't seem to be an area that attracts B2B.
I suspect it's because you have to be in the industry to understand the problem exists in the first place, so a huge segment of the YC applicants simply don't know about those opportunities as they've not worked in industry and only know about B2C opportunities.