If people are in receipt of benefits, then we're saying they don't have enough money to live off.
SO these people are due benefits but have claimed slightly more benefits than they were due.
How do you propose people pay back a debt, without the money they need to live off? And who is paying the costs of fruitlessly chasing people for money they will never have?
Now I get the 'they shouldn't have claimed fraudulently' and no they shouldn't but have you actually attempted to claim UC, it is by far the most complicated thing I have ever fucking done (and I am deeply regretting it so far as its brought me nothing but stress, which far outweighs the actual money!)...
There are many people who would fall into the fraudulent claim by genuinely misunderstanding, or by having a very 'grey' situation - for ex, a partner who stays with them sometimes, contributes zero financially to home or general outgoings, and lives elsewhere but isn't paying rent elsewhere as they are cocklodging with a parent (a situation many women find themselves in).
To the claimants mind, their partner does not live with them fulltime, doesn't contribute to the rent or bills, but to DWP's mind the partner isn't paying rent or bills elsewhere and is staying with the claimant a significant proportion of the time, so they'll assume claimant is lying and the partner is giving them cash.
The system does not allow for claimants who are stuck with partners who are arseholes.
You might catch and prevent a few who are actively and determinedly trying to scam the system.
But you would mostly be punishing people already in poverty, who have just fucked up or been screwed over by someone else, and ultimately it is going to be vulnerable people, mostly women, disabled people and children, who suffer.