You certainly can do somewhat useful AR on a phone. See e.g. some of the ML photo and sound identification tools (e.g. Merlin, Google Lens) that have very quietly gotten quite useful.

But AR that is really a killer app almost certainly needs a fashionable, light, easy to use headset and that's a hardware problem to a significant degree that probably doesn't have a quick fix. Yes, you can do something that's effectively a prototype that's more like a "headset" but that's almost certainly not a mass market device that most people will walk around wearing.

(There's also the social glasshole issue but my guess is that goes away with a useful enough device even if some people hate the idea.)