Under VFR sure, obviously under IFR they are looking at the instruments as much as 100% of the time, including the instruments which inform them if planes are nearby, and instruments which inform them of their location, direction, and all of the variables therein.

It would be madness if pilots had to rely solely on their eyes to locate other planes nearby. There is thankfully instruments which do this as well.

In VMC you still have a responsibility to see and avoid, regardless of whether you're on an instrument flight plan or not. (Ref: Regulation 14 CFR Part 91.113 (b))

Radar coverage has become ubiquitous in most places, but there's not universal coverage. Heads-up time is very important unless you're flying in actual IMC.

Can't find equivalent rule from ICAO, is this regulation US only?

Refer to ICAO Annex 11 - it's not the exact point being made by the GP, but note that traffic separation for IFR traffic from VFR traffic is only provided in Class A, B and C airspace.

Or, you know just flying the autopilot, which many do like 90% of the flying time...

I think what GP is trying to say is that because your average airlines has around 10+ km of altitude to loose before rapid disassembly commences, which takes a non-trivial amoun of time, compared to a car, which can go on a short and unintentional offroad trip within a few seconds or less.

No, you don't. you can fly over high mountains, having a lot less distance between the plane and the terrain, but how you lose altitude is more important: just a single km drop can be unrecoverable if exceeding VNE and losing a wing in the process.