Looks and readability matter in writing. I always uses spaces on either side of the em-dash; looks too crowded otherwise to my eye.

I've been doing doing it that way for so long I still often use the "--" construct from way back in ascii email days.

At least I've gotten past the two-spaces after a period I learned from the typewriter. Most of the time, anyway.

Every AI response I've seen that uses em dashes also includes spaces around them.

DFW famously put spaces around his em dashes. That being said I've noticed at least one major model seems to have started putting spaces around its em dashes.