There is no benefit for the Ukrainians to do this.
The benefit is to harm Russia at any and all cost and if they can achieve that, they see it as well worth it. Ukraine was never in a winning position yet they have been the ones committing war crime upon war crime upon war crime literally from the very start of this conflict (and depending on when you define it "beginning," they have been doing it from before the start and this is largely what necessitated Russia's intervention in the first place). Meanwhile, Russia has been highly, even shockingly restrained when it comes to taking actions with high potential to cause civilian harm. When you honestly compare how Russia has waged this war in terms of risk to civilian life to what the west (including Ukraine) has done in military operations and wars in the past handful of decades, Russia comes out as almost kind, looking like the benevolent "peacekeepers" that NATO always tried to paint themselves to their own respective domestic populaces. (This isn't to say warcrimes haven't been committed by Russian forces, particularly before Wagner was dismantled, but they are not systemic and are not at the scale of, for example, wiping out civilian infrastructure).
This isn't just a "Russia good and Ukraine bad" thing (though we shouldn't forget that current Ukraine is literally a Nazi-led project) but there are very obvious material reasons why this is the case. Like TreadOnMe pointed out, Russia came to the aid of what were essentially militias formed from Ukrainian civilians who were fighting in resistance of their own ethnic cleansing by the Ukrainian government. Russia knows that the territories it has been fighting over will be its responsibility to maintain and rebuild so destroying the infrastructure there and making enemies of the people who live there are not at all in Russia's best interests. This is a major stumbling block for the libs who constantly want to believe Russia is just a bunch of orcs hellbent on domination and conquest: material reality does not fit the idealist narrative they need to believe in.
Just because Ukraine commits war crimes repeatedly (as they have) and even as a normal order of operation, that does not mean that Russia will then be compelled to do the same as a tit-for-tat. There are certain lines that when crossed, Russia does have to respond to, but that doesn't mean they have to respond with commensurate cruelty to civilians. And they haven't.
Even if what you said is true, which it is not (see Formerlyfarman's comment), whether or not Ukraine was trying to ethnically cleanse the Donbas prior to the SMO (which it was), and regardless of the virtues or crimes of the people's militias, none of that matters as far as what I was saying. None of that has any bearing on the asymmetry of war crimes perpetrated by Ukraine vs the lack of them committed by Russia.