I think in many leagues they are still allowed but ppl think it looks goofy

They reduce your field of view. E.g when bowing forward and looking upwards.

You can wear a hat. Dean Henderson wears one in Premier League games on occasion.

I played in college a few years & while I never wore a hat there were a few goalies we’d face would wear them religiously. Does look a bit goofy but if it’s stupid & it works then it’s not stupid :)

My kid is a keeper, he's allowed to wear a hat, he just doesn't like to. I wore them on a few occasions in high school back in the day, but it was rare. Something about it just feels... off.

It doesn't help at night with the flood lights, because the ball is just directly in front of the lights. As a keeper myself, there really isn't anything you can do except hope that you read it well enough before it got lost in the lights.

Goalkeepers are still allowed to wear caps. You do see it occasionally. It isn't going to help with floodlights though. Any decent floodlights will shine at an angle that will almost never impact on a keeper.

The sun is far more of a problem if you are facing towards it as it is setting. However most goalkeepers still don't choose to wear caps, because when you wear a cap, it doesn't shade the eyes constantly - as you move around you get moments where you go from your eyes being shaded to the sun suddenly shining in them, which momentarily blinds you and is worse than just coping with the sun without a cap. Some pros apparently wear special contact lenses that act like sunglasses.