Aren't they both moderately overweight blond nerds?
Greg is barely a nerd, Theo is just uncool.
What can you remember about Theo's interests? He babysits Aster, and tries to not be like his dad. That's it.
Greg, as much as he is a loser on the bottom of the social totem poll, has interests. He regularly posts conspiracy theories on PHO, and he's a fan of gaming.
Everything about Theo, relates to somebody else. There isn't ever a "just Theo" thing.
Everything about Greg, is him being an uncool nerd, but at least we get personal interests out of him.
Of course overall neither are that great. Greg's biggest moments is helping out when Taylor is outed (and I don't even know for sure that happened, it's been so long since I've read Worm). Theo was effectively a protagonist for the Slaughterhouse 9000 arc and we still didn't see much of him as a person.
Realistically speaking you can role swap Theo and Aster and the story wouldn't change. You can replace Greg with any spineless introvert with morals/loyalty and the story wouldn't change.
But because they are so little, we can do so much with them. Stories with Greg as the main character, stories of Theo and dealing with his father. It's like that Phineas and Ferb episode where they made the Platypus toy, since they don't do anything, they can do everything.