The problem is that disappearing ink is a thing, and someone could swap out the source of ink (pen, stamp pad) in the voting booth.
Erasing is indeed a possibility with pencil markings, but this can only happen during the counting process - which should be open to anyone to audit, and anyone messing around with an eraser during the counting process would stand out like a sore thumb.
Where I have seen stamp pads used for voting, you do not take them with you in the voting booth.
You must press the stamp on the stamp pad at the official who gives you the stamp.
Stamping is fast and convenient. While corrupted officials could apply additional stamps during the counting, to make the vote invalid, that should be prevented by witnesses belonging to the parties that compete in the election.
Erasing is indeed a possibility with pencil markings, but this can only happen during the counting process - which should be open to anyone to audit, and anyone messing around with an eraser during the counting process would stand out like a sore thumb.