It's been a chicken and egg situation that's now coming home to roost (no pun intended).
For years, as long as people with children met some fairly low weekly hours requirements, they would receive quite a lot of tax credits, that made it not worth working more than the minimum, if they were in low paid work, especially if they had 3+ DCs.
My DSis does this. They have 5 DCs and she works 16 hours a week. Her DH is self employed so works the right amount of hours (about 8 IIRC) to maximise their tax credit claim, which is well over 1k pm. They own their own home outright, so no need for HB.
My neighbour also does it a single woman aged 50+. She spends an awful lot of time sitting in her garden drinking and gossiping with her DIL so I do know pretty much all her business and she has declared on many occasions that she would never work full time, its not worth it.
Lots of people are or have been self employed traders poking about at car boot sales to find bits to sell on ebay. They declare relatively small incomes such as 50 a week which is topped up several times over by tax credits.
Yes, the problem is that there are not enough jobs to go round and many do not pay enough, but it is also not right that people have been able to choose to work a couple of days a week in a relatively easy low stress job and be not much worse off than a professional on 30/40k pa who has qualifications, works long hours, travels and has to answer emails etc out of hour, etc.