I think a few factors and possibilities:
- As others said, many want to do therapy, not assessment. However, a caveat to another comment on here about programs teaching assessment, it is a requirement for APA accredited programs so everyone should be getting at least some assessment training, instruction, and face to face assessment work in any reputable grad program. So it's more interest not lack of inclusion of the training.
- You might spend 3-5 hours, maybe little more with a client doing the testing, then it's hours of scoring, interpreting data, and writing. I've had years of training in various assessment settings and it wasn't uncommon to see a full time psychologist focused on assessment maybe doing face to face work a 1-2 days a week and rest was scoring/interpreting/writing/backend stuff. I imagine many who go into this field want to "be in the thick of it" not behind a computer screen typing.
- The issue of school 'psychologists.' I remember mid 2010s there was a big thing in multiple states of psychologist professional organizations pushing to reclassify school psychologists who, aren't, in fact usually actual psychologists. But education unions and the school systems have quite powerful lobbying as well, so not sure what came of that. My points with this:, Firstly, there's a lot of subpar assessments out there by these school "psychologists" because their training is limited. They're basically masters level assessment clinicians with a focus on academic testing. Secondly, I'm sure some schools and supervisors see these other clinicians as boogeymen in the field and discourage those becoming actual psychologists from doing assessment (i.e. the school ones are often cheap, subsidized, or free as they often work for school districts and how can we ever compete oh no!).
- Cost of maintaining test materials, supplies, office space, as well as factoring in how much you charge vs how many hours it takes you to do everything you need from start to finish for an assessment. I'm sure some don't want to spend hours on the back end stuff for each assessment.