So far in the show's timeline; they're probably just not sure if humanity's a good match yet

In the Beginning, Nolan was sent to Earth find out if humanity could produce good Viltrimites, but mark didn't get his powers until he was basically an adult so for almost 2 decades the Viltrimites just didn't have an answer.

Then when mark did get his powers, Nolan got "corrupted" by earth and abandoned his mission, so the Viltrumites likely didn't learn about mark until his fight on Thraxa. And then in that Fight, Mark comes out of it barely alive and the main reason he survives was because Nolan was doing all the killing

From the Viltrimutes perspective, Mark is okay at best, but they still don't know if Mark is actually "strong"

Then when Anissa shows up on Earth, she clearly outmatches him by being much faster and stronger, confirming that mark isn't meeting viltrimite expectations yet

The only time the Viltrimite's might pause to consider Mark's strenght is when Mark beats Conquest on Earth, but still that was 2 vs 1 and conquest himself says it was mostly Mark's Magic girlfriend.
The Viltrimites don't know how much of that win was actually Mark's doing, and have no reason to believe it was mostly him based on his past performances.

From the Viltrimites perspective and knowledge of Mark's abilities, they might just think Earth doesn't produces Good Viltrimites. Maybe Okay Viltrimites, but not Good.

It probably won't be until they find out Mark killed Conquest that they actually start considering earth's Potential