Receipts are problematic as they enable compelled voting.
Obvs vote buying / intimidation could still happen but I'm not sure it would be practical at scale.
I could see the potential usefulness of a receipt that was in a locked drop safe for audit/internal control purposes.
I think it's also likely to cause harassment outside of polling places with gangs on sidewalks asking to see the proof that people voted for their candidate. You DID get a receipt, didn't you?
I've heard lots of stories where the head of the household fills in the mail in ballots for the whole family.
Abuse of mail in / absentee ballots are one of the few frauds that actually happen. The elderly and tight-knit religious groups are particularly vulnerable.