Because

1. Better a mined countryside than losing the war and
2. The countryside is already full of mines and stuff anyway.

As to other munitions: First of all those aren't exactly plentiful, secondly you'd also have to have the necessary hardware to have a similar effect on the battlefield. You can hook up a couple of PzH 2000, say, seven, put ordinary rounds in their barrels and tell them to all shoot say 10 rounds a particular area all 70 impacting at the same time (so that noone can take cover), or you can use one cluster round with a couple of duds in it. Ukraine doesn't just have a couple of PzH 2000s standing idly around just waiting for an opportunity to imitate a much cheaper cluster round.