It's not that YC specifically gets "preferred shares" -- it's that investors in general insist on liquidation preferences when buying non-liquid shares in unproven private companies.
How would an alternative scenario of investors buying common shares of illiquid stock in a private company actually be realistic? Maybe the startup founders could hypothetically insist on selling only common shares and never preferred shares as a condition of investment?!? But what investors (other than family relatives) would put in money in that case?
Or put another way, let's say we create a brand new VC fund to invest in startups and one of the novel concepts is that the fund only buys common shares to be more "founder friendly". The problem is that hypothetical VC fund will attract no rational limited partners with money because they know that startup founders can just take their invested dollars with no payback protection. Such a VC fund with no investors and no money to invest would be a moot point. The general partner of such a VC fund would be considered a "financial idiot" for buying common shares in startups.
In the end, the "preferred shares" is the market's "risk premium" that investors charge as an offsetting factor for losing 100% of their money. If startup founders can't find a way to convince investors to accept illiquid common stock instead of preferred shares, they need to avoid investors altogether and self-fund via bootstrapping.