We used tanks so extensively in Iraq that many are worn out and sitting in depot awaiting refurbishment. Field Artillery was present and used extensively in Afghanistan. As one battery commander told me, they moved the dirt out from under the Taliban.
We keep building them and then send them off to remote, dry, desert storage locations.
Where would you get an idea like that?
He probably got the idea from Obama, who made remarks along that line when declaring that we would never again need tanks because we (and likely most other countries) would never again fight a war requiring them and that acquiring more would be foolish... then a few years later the Ukraine and Syria brew up and demonstrate just how wrong he was.