Uploading passports, ID's and more to airlines?

Travelling internationally from the US after not having done so for about three years. I was surprised to get hit with a request to upload all of our passports, a second form of ID, and other documents to the United Airlines portal before travelling in a few weeks.

My immediate thought was: "No way in hell!".

I used to travel internationally at least once a year. Did so for probably twenty years. I'd just show-up at the airport with my passport and flight documents and that was that.

My concern: With all the data breaches and security/privacy issues, how intelligent is it to scan and upload such important and detailed information into an airline's website or mobile app?

Overthinking it?

Trust me, I'm an airline?

I am also wondering if this is a legal requirement these days or just the airline trying to be helpful? In other words, can you opt-out and just go to the airport with your documents?

And then the obvious question: Are they just going to scan the stuff at the airport anyway and store it in the same databases?