Because it is trivial to identify the people who vote by mail. This complaint comes up over and over and over and over and over again by people that have spent

Because it is trivial to identify the people who vote by mail. This complaint comes up over and over and over and over and over again by people that have spent zero seconds actually looking into how verification is done. You compare the signature with what's on the voter record. You check records to see if they passed away recently. Then you can compare addresses and any other personal details. This is more than enough information to strongly verify that the person who voted was who they say. It's why fraud is rare and why the cases of attempted mass voting fraud are immediately identified (see: Leslie McCrae Dowless)