Time and talent

Just because you're good at acting doesn't mean you're good at business arrangements, negotiation, scheduling, and finding opportunities. And once you start getting jobs you won't have time to look for the next one which could need to be lined up months out

The whole job of the agent is to look for and make arrangements for jobs. They have time dedicated to that task since they aren't trying to shuffle between doing the thing that makes money(acting) and lining up the next job, the thing that makes money for the agent is lining up the next job. They're also going to have the right network for it, again because its what they do. Agencies will also represent multiple people so while Client A isn't the right guy for every job they can still land jobs for Client A and use Clients B-Z for the other openings they're better suited for

Generalists aren't great at any task, this is why we tend towards specialists