Problem is the cam and pump MAY look fine to an untrained eye.
There may not be any chunks missing out of it, or obvious wear, and the angle you can see the cam doesn't make it easy to spot the obvious rounded shape of the lobes etc.... Forgetting about the rest of the follower which will have made its way around the engine for a minute...
Some people don't realise how little the pump actually moves and how much difference those few mm can make to fuel delivery etc.