WHY DO WE KEEP ON GETTING SA AS A SIDE PLOT

Honestly, it should be expected in omegaverse. It's part of omegaverse. The stories are usually writte with a focus on the alphas that control themselves around omega, for this very reason. There are exceptions, of course, but the "omegas exist to be raped by alphas" is a pretty integral theme of the general lore.

I think stories should really start being labeled as "No SA Omegaverse" for people who want to avoid it to be able to find it.

It is much less common than it used to though. There are a lot more non SA options for omega verse, now.