YC partners doing a lot of introduction during the fundraising process.

It puts them in a tough spot. If YC partner writes to an investor, this company is awesome, plz invest and then investor asks if YC is going to invest?

And they get back, you commit first and then we decide, it just doesn't look/sound good.

It is important to remember is that a lot of companies in YC batch fundraise on SAFE, so when a priced round comes along probably 90% of them are not even in a position to fundraise, because a product didn't work out, team issues etc.

Also, there is YC Series A program which basically is going to be the sign of approval for investors and I am pretty sure YC is going to follow in 100% of cases for that one.