If you’re looking for an in universe reason, while he could kick Thanos and send him flying, Thanos had multiple ways to prevent himself from being sent very far. Besides, I’m not confident that Antman could generate the force to send him far enough to matter even if big T didn’t resist. There’s a lot of mass in that giant leg, but not much speed overall. Being hit by a slow moving train still hurts, but it won’t kill you, if you see the concept.
Hell, Antman isn’t even that strong in giant size. While it isn’t in the MCU, most of the really strong characters in Marvel have gone up against the various giant ones. Thanos can hang with the Hulk, toe to toe, and Hulk has stopped a giant foot in the comics. While the MCU isn’t 1:1 with comics, it isn’t that far off
Don’t think too hard about the plot of any of the superhero movies, you’ll hurt your brain
To him it would be like trying to kick a spider, why didn’t captain marvel just fly him to space instead of ramming him into the ground is the real question.
