I'm not as current as I used to be with my military trivia (I blame getting older and getting to know more refugees, veterans and families who lost people in wars) but I'll have a go:

A tank weighs like 60 tons or so. The engine and transmission alone are heavier and bulkier than whole cars, so basically none of the infrastructure you have available in many car factories is dimensioned correctly. Modern armor is composite and includes stuff like ceramic components which you would not have the machines, processes and knowledge for. "Gigacasting" sounds impressive but it's "just" aluminium injection molding that can do relatively big and integrated parts and you can't just fill in some steel-composite armor material mix in the hopper and have a fully formed Abrams fall out the other end of the machine. Things like barrels are forged (I think), which you again would not have the right infrastructure for. And so on and so forth.

My guess would be: It would be more sensible to apply division of labor and - for example - have many of the car factories spit out CNC and cast parts that fit into their usual production envelope and are then integrated into other/bigger systems at your friendly neighborhood US armory (Krauss-Maffei or wherever, more likely), specalized stuff like aircraft parts from their Gigapresses, have them do electrical work for other systems, produce lighter (support-)vehicles, use their skills and infrastructure in quick mass production for things you really need a lot of (shells, basic supplies for your war-torn population's needs, and so on), have their prototyping labs work on more cutting-edge/improvised stuff like the Drones we see in Ukraine and Russia. I'm sure there are plenty more good (terrible) ideas to be had.