Mail voting already exists and works fine, despite the scenario you're positing. I live in Washington state where it seems to be the default.
I live in a country with only voting at the voting booth and I am perfectly happy with it!
In addition to this, the fact that you are not aware of voting fraud does not mean it does not happen. That's why it's important to care about potential voting fraud.
There was a proposal to install cameras on the voting booths, but it was rejected by the high court.
Surprisingly e-voting can solve these low-tech corruption problems, but it introduces a much larger attack surface.