I don't understand why they still have such thick borders, compared to smartphone screens that almost get to the edge. Anybody knows if there's a technical reason for it?

Tablets need an edge where you can grip it. Without thicker bezels, it’s harder to hold it without your fingers being on the screen. This is much less of an issue for phone-sized devices.

I don't think it is technical. Because of their size, they would be hard to hold without covering portions of the screen, if the bezels were thinner. As is, my fat fingers get in the way already.

To each their own, but I would rather have a larger border where I can rest my thumb without causing an accidental press/scroll a few times a day. The software-based rejection is not good enough and I am very willing to go back to the older look of the iPad if offered.

I already have a hard time not accidentally touching the screen while adjusting my hands' position or whatever with today's iPad's "thick borders"

I think it's an ergonomic issue. Phones (even the Pro Max size) can be held with one hand or two hands without resting your palm or pinching the edge to hold it. You could but it could cause some erratic behavior.

A tablet though doesn't hold well when just pressing on the sides. So having some place to grab and rest your palm is more necessary here. They probably could go thinner with borders but it's a balancing act of usability and aesthetics. Also have things like the camera to account for and on tablets you don't have to make a punch-hole or teardrop. The iPad Pro's also package in FaceID cameras so it could be a product consistency choice too.