Because "it's hard" and HMRC don't like "hard" things any more. They want the easy wins, i.e. where someone has sold a house for £1m a few years ago, but then dies and probate only declares £500k. It's an "easy win" for them for IHT. Land registry is clear evidence, then they just get the bank statements and can quickly pick the big outgoing transactions as to where the money went - just a few hours of office work!
In the same way, they've been targeting buy to letters where land registry and stamp duty return shows "Fred and Mabel" have bought a second house (stamp duty flags it up), so HMRC sit on the info for a few years, and then pounce when successive tax returns don't show any rental income declared, so they get several years' of back tax plus penalties etc. They "could" act within a couple of years of Fred and Mabel buying the second home, but they sit on it and wait for longer to maximise the penalties!
It's a lot harder when it comes to the black economy of "fiddling" taxes by cash in hand work, claiming a foreign holiday was a business trip, money laundering, etc., as HMRC don't have the same kind of "clear" evidence that they have from the likes of the Land Registry, DVLA, etc. Basically for things like child maintenance avoidance etc HMRC don't have the same "slam dunk" easy evidence and actually have to do some investigative work, like the way a tiny number of benefit claimants are caught because they need investigators taking pictures/videos of say disability claimants engaging in sports etc when they claim they can't walk more than 5 yards! It's just a lot harder! And the kind of people in the black economy know how to get away with it because they talk to eachother, share knowledge, etc.
It's a lot of why those engaged in benefit fraud/tax evasion, etc don't want to "settle down", i.e. don't buy their own house, don't have "fixed" business premises such as a workshop or office, etc - it minimises "formality" and the footprint evidence - best to have as little on official databases as possible!