Why do psychologists often not use mathematical models to express their theories?
Come read some cognition! It's a wide open field with angles on all kinds of psychology stuff, and we're having a great time throwing greek letters around like confetti. I think you might like it!
One of my favorite recent pro-model rants is this one psyarxiv.com/rybh9/ Although, to be fair they do argue for 'computational' rather than 'mathematical'. I would argue these both mean 'explicit formalism', which is what you're after here?
There's a whole journal of mathematical psychology
www.journals.elsevier.com/journal-of-mathematical-psychology
or Computational Brain and Behavior www.springer.com/journal/42113
is full of cool stuff on a surprisingly wide range of psych topics united by a common theme of awesome mathy-ness
Psych is very broad, but whatever corner of it you're interested in, there's almost certainly a math-y cognitive/computational version of it (as well). Those mathpsych folks definitely get around. Probably the most math-resistant corner of psych is social psych, emergent behavior of groups type stuff is absolutely vicious to model, as you would know, coming from econ... but see eg. cogsci.mindmodeling.org/2019/papers/0172/0172.pdf
Explicit formalism is the future of psych, and quite a lot of the future is here already