The election apparatus in the US is extremely hard to rig. It's run by local officials, so in order to fix the counting of voters you need to get to thousands of individual county/city/town election boards, all at once. Those boards have members of all parties generally present on them so there is a fair amount of local oversight to overcome, too.
There was a bit of time in the early 2000's where the voting machines themselves were suspect but some good work by independant researchers shined some daylight on that. Now most votes in the US are either done purely by paper ballots (counted by machines) or on machines that generate verifiable paper trails, and are very hard to just casually alter the count without being found out.
Republicans rig the vote by manipulating their media. Roger Ailes was one of Nixon's media advisors during Watergate. The lesson he took away was that if the media didn't hold Nixon to account, he would have never had to resign. Ailes went on to run Fox News in the mid-90s, and the rest is history.
You have to pick a leader somehow. In authoritarianism, the leader is often the one who can take over by force, and can maintain that force over time (even across generations, for hereditary systems). While it's possible for someone who takes over that way to be benelovent towards their people, it's far more likely they will be violent and overbearing, because that's how they got the gig in the first place.
And after a few generations, the one in charge won't have any memory of how their ancestor came to power in the first place, and just take it for granted that they ought to rule. So now you have a leader who is violent and overbearing, only because that's how their parent taught them to be, not out of any real experience accumulating power.