I know, I know, I feel like a chump even putting ten seconds of thought into this.

But Thanos has near God-like powers with the Infinity Stones/Gems. Even if Ant Man were ant-sized, Thanos would notice him well before dude got anywhere near his ass, even if Ant Man were riding a fly at the time. And without blinking an eye he would turn Ant Man to dust.

I mean, dude body slammed THE HULK with only one Infinity stone. There's no way that Ant Man would be able to get close to him.