Why context is the whole game
A scanner sees requests. An attacker sees a system.
Point a generic tool at your application and it sends traffic at endpoints. It has no idea which user is supposed to own which object, which role is allowed to call which function, or which two calls were never meant to touch each other. So it tests what is generic and reports what is possible.
The exploits that actually hurt are not generic. Broken object-level authorization, broken function-level authorization, tenant isolation failures, business logic abuse. Every one of those is a statement about your application specifically. You cannot test for them without knowing how your application is supposed to behave.
That knowledge used to belong to one person. A senior offensive engineer who spent days clicking through the application, reading stale documentation, and working out the roles and the ownership rules, until they held the whole thing in their head. Then they moved on, and it left with them.
Context is the difference between an attack and a guess.