The key phrase is “to make it worth it”. You’re swapping a DPS to specifically counter the Hammond. That has more consequences than the Ball vs Sombra matchup specifically.
For example, your team’s Tracer swaps to Sombra to counter ball, but their Tracer stays on Tracer. In this case you’re looking at more than just the Ball vs Sombra matchup - you’ve also got the Tracer vs Sombra. What if the difference between Ball -> countered Ball is less than the difference between Tracer -> Sombra? Right now the gap between Tracer and Sombra is massive, and the meta/stats agree that it’s not worth it to go Sombra even if it means letting the Ball be free.
I think you may have the cart before the horse when it comes to the meta. Meta is generally determined as what gives the best results most often - it’s not something that GM’s just sort of come up with to be dicks to each other (though admittedly being dicks is often a byproduct). Hammond shows consistent results no matter what he goes up against, whereas Sombra doesn’t despite how much she may or may not counter Hammond.