When teaching computer architecture, why are universities using obscure CPUs?

The answers don't really go into what I think is the main issue:

An introductory computer architecture course is generally starting from "here is what a transistor is" and hoping to end with "here is how basic computer games work" (e.g., nand2tetris). And has about 40 hours of instruction to do it.

Assembly is but a small part of the course. The ideal assembly language for such a course is one that is simple, something that you can let the interested student figure out how to implement it themselves with just a little bit of prompting. And frankly, most assembly languages aren't up to that task, even if you're willing to pretend that most of the ISA doesn't exist. x86 has issues with things like ModR/M or two-address instructions. ARM has things like r15-is-actually-the-program-counter-and-kinda-special. RISC-V coalesces most of the jump instructions into one or two instructions that is going to complete teaching subroutines in assembly (and also eschews condition codes).

At the end of the day, the stuff you're covering in introductory computer architecture is very far in almost every detail from how modern computers work, and it will take several courses on top of it to actually reach those details. The most important goal of such a course isn't to teach you how they work, it's to give you the information you need to understand the explanations of how they actually work.

One of the reasons why they used Scheme for MIT's 6.001 is because it was an obscure programming language. At the time, some incoming MIT freshmen were likely to have faffed about with computers in high school or before, learning languages such as BASIC or Pascal. Others would have had little to no computer exposure before entering university. Scheme, being a Lisp and having semantics that were quite different from BASIC, Pascal, or any other language such students were likely to encounter, allowed all students to begin on a more or less equal footing because the programming language used for pedagogy would have been equally unfamiliar to all of them.

Obscure CPUs have the same advantage. Plus with careful choice of architecture, or simply inventing your own, you make pedagogy much simpler because the architecture doesn't have all the complicated bits like branch predictors, register windows, speculative execution, etc. that modern CPUs have and you can focus on the basic, "must-have" components.

You can teach 8086 assembly which is still compatible with modern x86 and it only has like 20 instructions. It's lots of fun I made a whole bootable text editor an assembler and an almost finished lisp interpreter too.

mano machine :)

In my best dad voice... Son Im going to need to check your home work.

This sounds like fun little project to learn risk v with. It's on my to do list with a million other things.