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Because it arrives one small request at a time. Each is reasonable, each is agreed in a conversation, and none is recorded as a change.
By the fifth the project is a fortnight behind, and the conversation with the client never happens because nobody can point at when it started.
Raise it as a change against the project before assessing it. The record of the request is what makes the pattern visible later, even for changes you absorb.
Assess the effect on dates and capacity, price it at the contract rate, and put it to the client before it enters the plan.
A lightweight version — recorded, not necessarily priced.
The client for scope, the project owner for the plan.
Yes, per project and per client.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.