"Why are children of working parents in more need of a pre school education?"
They aren't. Quite the opposite, actually. The 15hrs universal pre-school provision is entirely designed to ensure that it is accessible to everyone, regardless of income.
You don't HAVE to take it as an SAHP. You can take it up, or look after your pre-schooler at home, or use alternative childcare (nursery, nanny, childminder), whatever you want.
If you work you can take it up, or use alternative childcare. If you work, you probably have to pay for the ADDITIONAL childcare you need to fit those 15hrs around your working week (so actually some children of working parents are LESS likely to have access to universal preschool provision given that most working parents need longer hours/school holiday coverage than pre-schools can provide).
Yes, for some parents those 15 hours help tip the balance of childcare being affordable or not, but they are not there to boost the country's GDP directly. They are there to make sure all children have access to SOME Ofsted-quality childcare and are prepared for education.