Because some of the windfarms still run on legacy contracts where they were incentivized by a guaranteed selling price for the power they produce. They are reimbursed for the loss of income they suffer due to curtailment.
We will be paying them £2.5bn a year to not generate electricity by 2030
https://www.nsenergybusiness.com/features/examining-challeng...
‘A wholly unsatisfactory state of affairs’ indeed
https://www.ref.org.uk/ref-blog/372-why-are-unsubsidised-win...