Flesh armies learn on the spot and don't have to be retrained when the enemy devices new strategy. Plus men with minor injuries keep working, while electronics with minor issues are just a bunch of useless steel (see any modern car or modern computer). Plus the opportunity cost, every factory making robots is not making shells.

Russian soldier put it better

https://i.poastcdn.org/73d5a79d8c6bf978f6bdee73b63c92a8a1370892e2d8a102b85fa464a44fd366.MOV