I dunno. I never had chronic back or neck pain until I started working office jobs. When I was at university, I spent periods sitting at desks, but was never strapped to one for 8 hour shifts. I would sit down at a desk for max 4 hours at a time apart from during extreme crunch periods, and would spend the rest of my day walking around, lounging and chilling in different positions, excercising, etc. As soon as I started working at an office it became noticably harder to reach an over-10k step count daily, and even though I continued going to the gym and doing heavy back days, my left trap has become completely hardened up, and I've occasionally had lower back pain too during stressful times. I'm totally unable to train upper traps because they are literally like bricks. When I have a weekend where I walk and lounge a lot, or am off sick, or go on holiday, my traps feel significantly better and my workouts are better. It's a lifestyle where you are unnaturally in one position for too long that causes this.

>I'm skeptical that there's anything especially unhealthy about sitting at a desk even for long periods of time. What's more plausible to me is that back pain comes from having a weak back

I figure few habits will give you a weaker back than sitting at a desk for longer periods of time.

Yeah I think it's exactly this.

A lot of people end up thinking it's about ergonomic chairs. An un-ergonomic chair will make things bad pretty quick, but supportive ergonomic chairs are part of the long term problem IMHO.

And deadlifts are not the only solution. I started out using just a backless stool and it helped a bit (can't slouch!), but what really sorted me out was using the "lumbar machine" at the gym for a couple of years. When Covid came along I couldn't go any more so I started doing "the plank" at home, that plus the "side plank" and some push ups have kept me going the last four years. And it's totally free and I can do them basically anywhere e.g. on holiday.