Voting by mail opens the door to massive amounts of voter fraud.  For example, a politician could go door to door, or go to a nursing home, to "help" frail or elderly people fill out their ballots and to make sure those ballots get mailed in time.  A union steward could demand his members bring their ballots to the union hall so they could all fill them out together "correctly" while eating beef stew and rice.  In the recent election we worried about illegal voters or electronic "hacking."  How easy is it for backroom bureaucrats to deliberately mis-count or discard mailed ballots they don't like?

House and Senate in Washington D.C. and every state require that every vote on any bill must be done in person on the floor of the chamber -- legislators cannot stay in their offices, or back home, while voting electronically or by email.  Legislator on deathbed in hospital got wheeled in on a gurney to cast his vote in person.  Why?  To protect against fraudulent voting.