I do a lot of motorway driving and have done for many years. To answer the OP's question about why 80/85 etc... I think many people drive at that speed because (at least to me) it feels like a much more natural speed on the motorway. 70 feels slow... 90 feels too fast... I think 80-85 is a natural reasonable speed - IF the road conditions are safe - and that is the key with motorway driving.
From my experience of motorways, there are far too many people who have never learned how to read the road - both around them and ahead. If people paid more attention to what was going on around them on the road - and were properly reading the road ahead - there would be far less accidents.
If it is a dry day, road not too busy and I can clearly see ahead etc... then I'll drive faster. If the weather is awful, poor visibility, surface water, fog, or whatever... AND/OR the road is busy, lots going on etc... I'll slow it down and maintain an appropriate speed for the road conditions.
What I find terrifying on the road are people who pootle along in the inside lane at an unnecessarily slow speed clearly oblivious to what is going on around them - including other cars joining the motorway. You see them - just pottering along at 40 or 50 when there is no other reason to drive at that speed - they aren't looking at what is going on - they are only focused on the few feet in front of their car. It's so dangerous and I've seen more accidents and near-misses as a result of that kind of driving than from people doing 80 in good road conditions. I do distinguish between these people and drivers who may HAVE to drive slow if, for example, they've had to change a tyre & are driving on the spare or whatever. You can tell the difference - the dangerous ones are off in their own little world and have a terrifying lack of awareness of other drivers or any issues in the road behind or ahead (lane closures... emergency vehicles etc.). Slow doesn't automatically mean safer.
If I'm joining the motorway, as I'm on the slip road, I'm already reading and assessing the road to determine what is going to be a safe speed... I've already picked up on how the motorway is "running" - ie. does it feel more congested than usual, are people driving weird (after lockdown ended it was like everyone had forgotten how to drive and there was all kinds of weird behaviour going on!), is it running "fast" or more stop start... who else is on the road - lots of lorries or what? I'm looking at everything going on around me - and it's scary how many people don't do this.
Another reason I prefer to drive a bit faster when conditions permit is it allows me to get out from behind lorries in lane 1 - and gives me better visibility of the road ahead - I can see what's going on far further down the motorway and adapt my driving accordingly. It's not about driving faster to be reckless - it's about what feels a natural driving speed and about actually getting away from drivers I consider to be more of a hazard. Creating distance and space around me on the road - it's a more enjoyable drive and safer.
I admit I've been on more of an eco drive lately with the fuel prices but it's from an economic perspective than because I consider it better to drive that way. I still adopt the same practices though and will always adapt my driving to the road/traffic conditions first and foremost.
In terms of speeding in towns etc... I just don't do it. I don't speed around built up areas or on country lanes etc. That to me IS extremely dangerous and reckless.