Because there is a maximum sum that can be paid per household dependent on the oldest occupant. Say you had a couple aged 90 living with their 70 year old son. Not unfeasible. Nobody receives any benefits other than SP. They would each receive £100. If the son wasn't there, the couple would receive £150 each.
WFP is not deemed a contributory benefit so the argument of paying in and getting out can't be made. If it was deemed contributory, it would be paid out of the National Insurance Fund which is currently awash with funds.
WFP was introduced by Gordon Brown in his 1997 pre-budget statement as a two year temporary measure to help pensioners. It was paid £20 per household or £50 for households in receipt of Income Support (the forerunner to Pension Credit).
For this winter and next, every pensioner household will receive £20 extra to help with their bills and every pensioner household on income support—nearly 2 million households—will receive £50 extra. The cost will be met from reallocating the savings on our contribution to the European budget.
hansard.parliament.uk/commons/1997-11-25/debates/e9c29db6-c1fb-4840-b5d5-1760462a5855/Pre-BudgetStatement
Meantime, Harriet Harman was charged with increasing the uptake of Income Support, same as the withdrawal of universal WFP last year was meant to do for Pension Credit- with limited success.
It didn't work in 1997/98 either so the WFP became a universal annual payment, increased to £100 per household in 1999 and £200 in 2000. The extra £100 for older pensioners was introduced from 2003. In real terms it's worth a lot less now than it was.
A briefing paper was published on in November 2019 (just before the 2019 GE) setting out the history, the many debates over poor targeting and options for reform (and the problems of each option). See right at the end of the paper. Having tried option one, the government is now doing option 3.
researchbriefings.files.parliament.uk/documents/SN06019/SN06019.pdf
Remember that Theresa May's election manifesto did include a proposal to means test WFP.
The options from the 2019 paper were never acted on by the Johnson government not least because the government was focused on Brexit, the pandemic and then the cost of living crisis with rocketing fuel prices. Pensioners were given extra help with a £300 per household boost to WFP for 2022 and 2023 to £500 and £600.
For all the furore last year over WFP, the new government was doing what has been proposed by the old government seven and five years earlier. People can make of it what they will but it wasn't a new idea. Poor targetting had been discussed for many years before that.
Anyway, I have gone off on a tangent but the main points - it has always been a per household payment and a non-contributory benefit.