Because it's bad for their side. Mail in voting massively improves Democratic turnout for a lot of reasons. And when Democrats actually turn out, they win every time. It's the same reason they support gerrymandering and voter ID laws. It has nothing to do with what's "fair" or "democratic" and everything to do with ensuring their ideology doesn't have to hold up in an actual fair election.
Here’s the Brookline’s Institute, a pretty neutral bunch, on fraud: https://www.brookings.edu/blog/fixgov/2020/06/02/low-rates-o...
Some analysis from first principles on both fraud and bias: https://fivethirtyeight.com/features/there-is-no-evidence-th...
This non partisan group did detailed analysis of two elections, all of Colorado in 2014 and of Utah in 2016 looking at turnout and bias. https://voteathome.org/
If you want more academic work there’s quite a bit of it done by economics and political science profs. Just search for “mail vote bias”