Series A today requires minimum $1m/year in ARR. That's far beyond what YC companies are able to achieve in a few months. So they are raising at a Seed level

Series A today requires minimum $1m/year in ARR. That's far beyond what YC companies are able to achieve in a few months. So they are raising at a Seed level which YC companies get a good deal on because YC is effectively running their fundraising for them e.g. Demo Day, vetted investors, optimised process etc.