Good candidates in lower orbits were all short-lived and reentered long ago, and possibly weren't sufficient to test the claimed performance, to start with. What even makes you think they "haven't bothered"? I have no idea why they didn't launch a dedicated target though - might have been budgetary reasons, bureaucratic failure, or because of a specific test envelope. Alternatively, the military rarely care when it involves strategic matters, you can complain but it just works this way. In particular US military who blew up the Solwind observatory at 550km to test the ASM-135, the only US ASAT weapon currently in operation. It was in 1985, the same time when USSR was developing its counterpart, so it's easy to posture as morally superior when you already did the damage and there's currently no need for another ASAT weapon.
Edit: here's a better analysis than most: https://spacenews.com/op-ed-lessons-to-learn-from-russias-nu...
(meta: threads like this usually have little substance and garbage quality for HN standards. People start assuming all kinds of things and post kneejerk reactions without even bothering to research what's publicly available. I typically avoid posting in these, but then they just become outrage bubbles)
The last few weeks have had some extreme rhetoric coming out of the US, claiming that Europe is about to be invaded by Russia, so extreme that even Euro allies find it a stretch.
Maybe this doesn't make the news for many Americans but in Europe it certainly doesnt go unnoticed when powerful governments talk about full scale war on your doorstep.
Now there's one country on Earth with a magnitude more spy sats than anyone else, let's call them X they are making these claims against country Y, in response Y demonstrates its ASAT capabilities for the world to see.
Is it really that hard to understand?
https://time.com/6116867/us-warns-russia-invade-ukraine/
https://www.msn.com/en-us/news/world/amid-tensions-with-russ...
https://www.cnn.com/2021/11/05/politics/bill-burns-moscow-uk...
And it still doesn't answer why the Russians chose to destroy a huge sat (more fragments) built by the USSR (more material to fragment) at a relatively high orbit (more time for fragments to destroy other sats). They could have done the thing India did and ram a small-ish sat at a low orbit and the fragments would be gone within a couple of months.
Instead they chose to even endanger their own cosmonauts on the ISS.