YC investors, like all other investors, are only interested in making money. You don't make money by paying people twice as much interest as everyone else. Something is fishy.
Marcus by Goldman Sachs claims their rate is "8x the national average" at 4.40% and I'm sure they're making money just fine too.
The national average is only 0.46%, according to https://www.marketwatch.com/guides/savings/average-savings-a...
I checked many other sources, all reporting in the 0.45-0.6% range.
Meanwhile, there are plenty of banks that offer 5% or more. So merely being twice the average is actually still pretty low.