Basically because of ballot stuffing, booth capturing, ballot tampering, fake ballots etc etc. Paper ballots have been a major vurnerebility from experience of several decades in rough and tumble politics. The article on EVMs I linked has descriptions of it. The EVMs rate limit voting to prevent this from being a major motivation for political parties. The approach that seems to be tried and tested and working well seems to be to use dumb unconnected and robust electronic voting machines which can't be hacked without physical possession and the using massive security to prevent physical possession to attackers.
The problem is they have no protocol for better protecting the ballot box and responding to ballot stuffing attempts and incidents. If you're not going to protect the ballot box it doesn't matter what tech you use for voting.
Only auditing the paper trail when the electronic results seem suspicious is a vulnerability in my opinion.
All these technical solutions target the non-problem of counting results quickly. You can scale up paper ballot counting by including more people, which also strengthens trust in your democracy.