I would like to understand why do I need to use
at with games?
He would be better at tennis if he practiced well.
Why I used
at here and not
Why do good, better, best, bad, worse, and worst license a prepositional phrase headed by at?
The preposition at brings to the fore the sense of focusing one's energy and attentions, thereby casting its complement as something requiring such focus, an activity.
If a site manager asks a shift supervisor about a spill that needs to be cleaned up pronto, the supervisor might tell the manager:
We're hard at it.
meaning, we are devoting our energies and attentions to that activity now.
Tennis is being cast as an activity by the preposition at.
"Better" licenses a prepositional phrase headed by "at" as a complement.