Why do you believe this?
Because there are too many checks and balances involved to "rig" an election, particularly state and national elections. I volunteer at the polls and I vote. One of the things I do is play watchdog to ensure precincts are running smoothly, people aren't being denied access to vote, etc. There are hundreds like me who do this.
The media is all over elections and the tabulation process. There are constitutional/statutory provisions for recounts. The courts can get involved. Just way too many safeguards in place to make the blanket statement that "U.S. Elections are rigged."
Why do you say they are rigged?