How does Pichu get a hat?

It puts it on

But, seriously, that's weird, I just got two more today. Dunno if eggs acquired before the event started get no hat

The pokemon on the eggs are determined at the time that you receive the egg, be that from a pokestop or whereever.

So a Pichu without a hat is from an egg acquired outside the hat event. Any eggs which came before the current event may have a Pichu, but it will not have a hat. Any eggs picked up during the event but hatched afterwards, regardless of how long afterwards, will have the hat.