As someone else said, I'd have had no problem evidencing what it would have been spent on - in theory.
But working ft and parenting a child 24/7 this would have laid more on to me, having to keep receipts, work out what I'd spent where, what proportion of that came from the CM and if I'd spent the CMS on the right stuff or if I'd say paid for the nursery with my wage when it came in, but then used the CMS to pay the rent, because the timings coincided, only a proportion of which should be paid by CM, or the freezer breaks and so I use the CM to buy a new one but support the child myself for the rest of that month etc?
And if I put in a receipt for 'clothes' or 'haircut' how do they know that it's for me or the child?
What happens if it's deemed I've spent money on something I shouldn't have?
Tbh I wouldn't bother chasing it if that was a thing, he only had to pay a few quid a week and we never got that because they 'couldn't find him' so to go to those lengths to be a few quid up, although it would have made a difference, just wouldn't have been worth it.
And I really do think that it would give these feckless absent parents a weapon, they'd be constantly 'reporting', every time the RP went out without the child, every time they got a hair cut or had a new pair of shoes, it'd tie everything in knots.
And you'd then have people demanding it's done for child benefit, then benefits like UC and disability benefits and then pensions.
If a man demanded that his partner accounted for every penny of his money in this way while they were together, people would probably think it was financial abuse.
ETA - the only way to change this and apply pressure to the right places is social change, until it's socially unacceptable there's no impetus to change the way it works.
Look at smoking, was everywhere 50 years ago, popular, cool even. Advertising everywhere, there was barely anywhere you couldn't smoke.
Now it's the complete opposite, it's socially unacceptable, and bans were brought in in response to that, advertising bans, packets behind screens and not deemed attractive, no smoking in public areas and a real recognition of the harm that was being done, and people vocal about it and criticism openly of smokers. But people's attitudes had to change first, then pressure was applied, then the shift happened.