I think it's down to incompetence in senior civil service and governmental levels.
I'm pretty sure that no one thought of the consequences/implications of the £50k threshold for child benefit and £100k threshold for childcare being based on individual income rather than "household" income as used for benefits. Just different governmental departments not talking to eachother and not bothering to check implications against other factors. I.e. benefits system is normally "household" income, but tax system is normally individual income. Some pillocks in government started to mix the two without understanding the consequences. I.e. child benefit is a benefit, but is repaid if necessary via the tax system!
Hartnett, an ex HMRC chief tried to justify his departments' incompetence by citing the sheer size of the tax statute and saying no one could possibly know and understand it all! What a cop out!! It's their bloody job to understand it and it's not as if it's just one bloke on his own. HMRC and the Treasury have a small army of staff, so there should be people who know their own separate bits of it and then actually communicate and liaise with others!!
A new cock up occurred just this last week. HMRC decided to change the benefit in kind tax rules on crew cab pickups which they thought, correctly, was a tax fiddle as they're taxed as vans (cheaper benefit in kind tax), than cars. Made big announcements, clearly spent a lot of time changing the rules, setting up transitional arrangements, etc., Then a couple of days ago, the Government vetoed it and said the old rules would continue, i.e. a low tax perk!! Sunak only did that to avoid criticism from genuine business users of pick ups such as farmers!! You'd have thought that someone within the Treasury would have flagged it up and stopped HMRC making such a big policy change without consulting government?
When you see this kind of fiasco, you start to realise that there's still no "joined up" thinking within the government nor the governmental/civil service departments. You start to realise that no one actually thought through the £50k and £100k thresholds and clearly didn't consult with other departments affected. There should be proper due diligence etc for all such changes, but clearly HMRC just ran roughshod over procedure. What's the betting no one in HMRC will get disciplined for such a mammoth cock up - they'll probably get promoted!
You can also understand how, during covid, we ended up with the covid support scheme fiascos that left over 3 million innocents excluded but at the same time allowed fraudsters to claim billions in grants/loans to which they weren't entitled and which the govt will never recover - all because of the same lack of joined up thinking, lack of proper due diligence, etc.