YC is still YC. Investor's come to YC to win, and win big. They want to see companies shooting for the moon, not worried about profitability 12 weeks in.
12 weeks in validates a market.. and proves you're easily displaced
May not matter if there's cash to make off with. Plenty of folks don't care about long-haul world domination or owning a market; if being a Significant Player for 3 years will earn them enough to retire, why not?
That's fine for you and many others. However, it's an anti-pitch for a YC company: demo day is really auction day. They're selling a chunk of themselves to an institutional investor sifting for disproportionate returns.
Yeah but it hardly ever happens does it; I'd love to get 12 solid weeks to show what I can do but I'm stuck trying to build and launch something quite complex in my spare time, it's really difficult so we've taken ourselves to Lanzarote for a full week developing, which we are half way through. You quickly realise that a few issues and a few nights out and your week is over :-/