What is it about damage spells in pathfinder 2e that’s considered ‘weak’?

I’ve read repeatedly that casters in PF2e are mostly relegated to support roles, i.e. buffing allies, rather than dealing damage. I know that this was an effort on Paizo’s part to deal with the linear fighter/quadratic wizard problem.

But I don’t fully understand why damage spells are considered suboptimal. Is it because casters don’t get access to items that improve their to-hit chances on their spells, the way physical-damage-dealers do with e.g. +1/+2 weapons etc.? Or is there something else in the mechanics of casting damaging spells that I’ve missed, which is a problem?

If the former, would a suitable house-rule be to allow weapon potency runes to affect spells cast while ‘wielding’ the weapon? Or add equivalent runes to the effect of, "while wielding this weapon you gain a +X bonus to your spell attack bonus and spell DC"?