> I’ve always hated the 1 rep max mentality. Mostly saw it in high school with kids trying to out do each other, but it fails to promote strength.

I agree. There is a difference between the mentality of high school lifters chasing a 1RM to impress people (often called "ego lifting") and the use of the 1RM as a baseline for measuring a person's strength for the purposes of developing a strength training regimen.

The 1RM also takes into account comfort of the lift. If you feel like your spine is going to snap, but you complete the lift then the weight is probably too heavy, for example. Some lifters use the Rate of Perceived Exertion (RPE) as a heuristic for this [1].

[1] https://blog.nasm.org/rate-of-perceived-exertion

Knowing your 1RM is a fundamental necessity in non-RPE based strength training programs. It's how you safely and efficiently progress in your lifts. RPE programs are based on perceived exertion for a variety of reasons, but I've never met an RPE lifter who didn't also have a very good idea of their 1RM, and most still regularly tested it.

And if you're not doing a program, you're not training. Which could be fine! But training is different than just exercising.

> I’m all for going slow. If 5 reps are better for you then great, do 5!

There's lots of underlying science around this - it's not just "what works for you," but instead what works best for the overwhelming majority of humans. If your goal is strength training, biasing towards fewer reps of higher weights is basically settled science. If your goal is bodybuilding, higher reps of lower weights is basically settled science. Though, 10 reps is in-line with plenty of strength training programs, particularly for intermediate lifters - it's in a range where you get solid results for both strength and hypertrophy. And, frankly, most powerlifters are lying if they say they don't want at least some hypertrophy.