Cluster munitions certainly can take out tanks, but not all cluster munitions are made equally. There are several types. Just a quick Google search brings up the CBU-100 Rockeye II which is an anti-tank cluster munition.
We’re also sending them these because cluster munitions are no longer used by the US military. So it’s decomission or send them to be used. It’s cheaper to ship them then dispose of them.
And we haven’t ran out of modern munitions. We’re running low on AVAILABLE SURPLUS munitions to send them. But we are ramping up production to replace current stocks.