…because it is easy. Because it is easy, Democrats tend not to worry about the votes being insecure. You can't think of how Democrats could cheat because you don't want to know that they can cheat. But history shows that everybody cheats if they can. That's why the rules have to be aimed at stopping cheating.
It was the GOP that first put forward the idea of easy to get mail in ballots, especially in Florida, because they figured it would make it easy to see who voted and who hadn't and nag the folks who haven't. Which is true. Problem was, the GOP found out that 1.) The Dems used the mail in ballots more than the GOP did, and 2.) The Dems were as good or better at nagging people who hadn't sent in their ballot yet. In the meantime, the GOP in North Carolina (and, perhaps, other states) was getting caught filling out mail-in ballots for voters. So, the GOP changed its tune and suddenly decided that easy mail-in ballots weren't so good for them, not because it hindered voting security but because it increased the opposition's access to the ballot and they got caught doing some of the very shit you're claiming will turn elections around.
So, contrary to your assertions, we know it can be exploited...the GOP tried it and got caught. If it was as easy to exploit and stay out of jail as you believe it is, you and I both know the GOP would be changing the law to hand out ballots on street corners.