I agree, anyone should be able to learn magic. I would love to have some of my werewolves learn magic, Remus Lupin is one of them. And part of the reason why I play very few aliens is because they are blocked from all the other occult lifestates.
I think the reason why this is impossible is the sims 2. That game allowed hybrids to the extreme. The devs even made special servo versions for servo vampires. With two or three occults that's doable but at the end of the sims 2 the amount of possible combinations became too complex. You could have something like a zombie-plantsim-werewolf-wizard. That's why sims 3 and 4 don't allow it. You can of course cheat it or create one through a glitch with the dark leyline lot trait.
I understand that all combinations are impossible but magic should have been an exception because it partly feels like a skill.
I personally stay away from them because I am afraid to risk my long term save. If you can and want to use mods, there is one created especially for hybrids.
https://modthesims.info/download.php?t=643617&cb=1726968908