Many of a cat's teeth are just too small and narrow to drill and
fill, they'd break off -- and there is very little space to attempt
to work on the not-very-much larger back teeth.  If you've never
seen a dentist struggle to fit his big fingers and instruments to
fix a young child's back molar, perhaps you can imagine trying to
fit gloved fingers and dental instruments in a cat's mouth, about as
small as the opening of a shotglass.

It's probably also less risk to the cat to quickly remove the tooth,
than to keep him under anesthesia for the length of time needed to
attempt drilling, cleaning, sealing, filling and shaping.  And you'd
have the additional risk that the same tooth might still decay along
the seam of the filling and need another anesthesia procedure to
extract it later.