Why are there sewer odors coming from my shower drain?
You have sewer gas. Water in your plumbing traps is supposed to prevent that. There's some problem that is resulting in not having a water seal in your traps for these items (they may share one trap if it happens to both at the same time.)

If you pour water down the affected fixture (slowly) does the odor stop shortly thereafter? Or, if you carefully do this after each use, does the problem not occur (since it can be hard to tell when an odor stops coming in, if it's lingering around the bathroom.)

If so, you may have a venting problem, with the traps being sucked dry when lots of water goes down and creates a siphon (what venting is supposed to prevent.)

How old is the house and/or the bathroom? Another common possibility for an older house is that an old drum trap has failed, and is not functioning as a trap at all anymore. "Pouring water slowly down the drain" would have no effect in this case.

It's surprising that plumbers would not have mentioned or investigated these possible causes, unless they were only asked to clean the drains, rather than to identify and solve the smell problem.