We have detected the effects of gravitons (...if it's gravitons and not something else) at macroscopic scales, however we have not done the same with monopoles. So that comparison is not apples to apples.

I have not heard anything about the SM predicting them being impossible, but I assume it does given that I haven't heard anything about the subject. Source: I have a master's in physics and a friend of mine did his bachelor's thesis in monopoles in classical electrodynamics (albeit modified) and his master's in quantum ones, so he would've told me if he knew.

However, if any GUT theory we have come up with is correct (because a few exist), then that does imply the existence of monopoles.