Apple does do keyboard only repairs; but Apple has a policy that they never do partial repairs. Your laptop is sent back as it came, or fully repaired.
I’m not saying it’s right, but drinks trigger small indicators inside the chassis warning the technician that the board may have taken liquid damage. From Apple’s perspective, even if the board appears to be still functioning; it cannot be warrantied and should not be considered reliable (to them, it’s as good as broken).
On Apple’s scale, I do somewhat get it. You don’t want to give someone back a “repaired” laptop that burns out later from latent damage. But it’s still… expensive and overly cautious.
Edit: I guess it’s somewhat a matter of perspective on what the repair is meant to accomplish. For you, it’s “I want a working keyboard.” For Apple, it’s “fix all potential consequences of water damage and make it safe to use again.”