YC's cheque size is $20,000 at max. Most venture firms that have > $100M under management won't write less than $1M cheque. Why? Because that's how their business works. So for institutional investors, it's a huge risk to back someone random whom they don't know. That's why YC is successful.

Nowadays, VCs are rarely the first money in bank. So YC and other accelerators act as a screening step for most institutional investors. And VCs can be successful by writing larger cheques at a later stage. So win-win for VCs as well as accelerators.