I generally don't like controling more than 1 sim, so I love the roommates feature! The problem I have is that outside of university, the Sims who I ask to be roommates constantly leave and come back into the house. They just stand in the room, then stand outside, then stand in the room, etc, constantly.

I read somewhere here that they need to have jobs to live a normal life, so I checked to be sure the NPC had a job and he does. He still leaves and comes back. What gives?

I've tried:
- turning a household Sim I created into an NPC using NRAAS (i have 2 avatars, only 1 is selectable)
- asking an NPC to be a roommate using the interaction menu (just 1 avatar)
- adding an NPC to a household, then selecting "make NPC" (2 avatars, 1 selectable)

They all do the same thing.

I've done this on PC, I believe, several years ago – I had a big house and made 4 rooms, each room had specified Sims allowed to enter, and 3/4 were not controllable though they all appeared in the sidebar. I haven't done it successfully since and usually give up.

If it matters I'm using an upgraded 2011 MacBook Pro with the latest OS, the game and expansions purchased through Origin.

This is a commonly enough reported issue with NPC roommates. Their autonomy isn't always the greatest, but I've only ever seen it as bad as you describe in Uni World. I was going to just leave at that until your last line -- aha! The Mac version of the game can only utilize 2 GB of RAM no matter how much you have installed. It is very possible that the broken NPC autonomy is a by-product of an already over-stressed game. We already know that Free Will for our active sims goes out the window when this happens, so it could be a similar thing. It doesn't take much, especially with all or most of the EPs, to get close to that RAM limit and then the game just cannot have any more.

Can you see from Activity Monitor how much RAM your game is using after it's been loaded and running for a while?

Ahh that makes TOTAL sense as to why I could play properly on PC! As I posted my question I figured it had to be something like that, because the Mac version is really just a weird hack. I monitor my CPU but not the RAM. It uses half my CPU, I didn't realise it caps out at 2gb of RAM.

I might just have to run it on PC to ply how I want... or just let the roommate walk in and out and try not to let it bug me. I got the roommate I wanted because random roommates kept showing up and he was one of them, I just let him. he leaves a bunch but also does some tasks so it's not sooooo bad.

It shouldn't be difficult to casually check the RAM usage occasionally without formally monitoring it. You could minimize the game and bring up the Activity Monitor at various points to see around where the usage is hovering on TS3 in particular.

It's also considered helpful to purge (reset) RAM frequently throughout gameplay, especially if you are hovering above 1.7 GB or even closer to the cap.
https://bluebellflora.com/resetting-ram-during-gameplay

Personally I gave up on this borked version of the game years ago and play TS3 for Windows on a Bootcamp partition instead. Although of course not everyone has a Mac with enough drive space or enough patience to manage two operating systems.