Not switching lanes thing - this goes back to the driver's ed course I took like 50,000 years ago, but the gist was auto collisions pretty much never happen when everyone is just cruising and staying in their lane; collisions happen during transitions like lane changes, merges, accelerating/decelerating etc. So, best practice in multilane highway situations is to get established wherever you're comfortable among the 'pack' of cars cruising around you, and then maintain that position relative to the other vehicles. So when douchebag decides to ride my ass, me changing lanes to get out of his way compromises my own safety. Tailgaters tend not to be very patient, so responding to their behavior by instead decelerating (slowly), they usually don't make it more than a few mph before they give up and go around.

Why are people tailgating when there are multiple lanes, unless you’re in the leftmost lane for too long?

Power move is my guess. I personally live in the cousin-fuckingly deep US south, and I drive a tiny car. 90% of the time someone wants to tailgate me, it's some lifted monster truck... I think rednecks are actually offended by little cars. The other 10% are fancy sports cars. In either case, it's the kind of vehicle that screams insecurity, so trying to impose themselves on other drives fits the M.O.

I tend to cruise in the middle lane - that way I don't need to worry about people merging on the right, nor pay much attention to traffic behind me to ensure I'm not impeding the left. So, it's usually in the middle lane that I have issues with tailgaters. If I'm cruising in the left lane, it's usually because EVERY lane is jacked up, but in that situation I'm not getting 'out of the way' of the people behind me cuz I'm stuck in traffic right with em. People still tailgate in that situation and it's like... bro I get it, I wanna go faster too, fuck off.

Tldr, people are assholes.