Because you're not sure what exactly Atlas/Hercules is supposed to be holding up. Is it the dimension of "space" around Olympus? Is it the actual entire "universe"? Is he holding up Eternity? Why? That doesn't make sense.
To me this feat is the same as Thor's lifting of the Midgard Serpent. There's no way to quantify what the feat entails. It is a literal lifting of physical mass? Or is it a metaphorical? Or some combo of both?
Although, I don't see how the ground between Atlas and Hercules remains intact if they are literally pressing the mass of the universe. Kind of like the Spectre landing on the Moon and the Moon not shattering. People here invoke that all the time to prove that Spectre isn't all too heavy, and thus Superman and Wonder Woman carrying him isn't all too great a feat, but are not doing the same with Hercules' feat, which confounds me.
Also, I find it odd that people are using this unprecedented feat to tier-up all the characters who have matched-up with Hercules, but do not do the same for Superman's peers when Superman executes an enormous, "ridiculous" feat. This also confounds me. Of course, if this feat tiers-up Marvel characters, then, to be fair, it does so for DC characters, too, by way of the Marvel/DC crossovers.
Personally, I don't subscribe to that point of view, but fair is fair.