That's not true. An angel investment after demo day is worth a lot less equity than the YC investment. Most YC companies raise their seed rounds at $5 million - $20 million valuations, so it would take many hundreds of thousands or even a couple million to end up with the same 7% that YC gets.*

That kind of money would usually be put in by a larger fund. Most angels write checks for $25k-$100k.

* One difference being that the demo day investors will have preferred shares upon conversion and YC takes the same common shares that the founders and employees get.