That's because the polls reflect a more organic public opinion i.e., how people feel based on their everyday lives. But that is of little to no concern to any political party.
Their main concern is keeping the likes of Murdoch, Rothermere, Barclay, Mullen or those UHNW individuals who have their ears onside as they know, that come election time, it is those few who will control the narrative and decide who gets elected.
All it takes is for the Mail / Times / Sun / Express to run few well placed campaigns and enough of the electorate will be manipulated into voting the way they want them to and sway the election result their way.
Brexit was as overt as you could possibly get about it. I've already spoken about how they framed the immigration narrative to favour leave but my favourite example is the bendy banana campaign from bojo, the sun, the mail, etc e.g., EU bans bendy bananas.
The reality is that the bendy banana legislation came from COPA COGECA (an agriculture & food producder collective/body of which the UK were part of at the time) asking the EU to legislate to help improve on the quality of bananas grown by EU (primarily in their overseas territories).
The goal being to make them more competitive/first choice in the banana market (the industry were quite persistent with their lobbying on this). The EU responded and published regulation 2257/94 which stated, amongst other things, that bananas should meet minimum quality standards. It never banned "bendy bananas" from being sold but did classify them into various categories with only some being eligible for sale to the public.
It is also worth reiterating that not only did the EU agricultural industry (including the UK representatives) ask and lobby for this, but they were also heavily involved in writing the directive too! Oh and all this happened in the early 90s, with Regulation 2257/94 coming into effect in 1994, before being dropped in 2008 (I think).
Yet come brexit time all this was ignored, BoJo used it as an example of why leaving the EU would be great for the UK, and the pro-bexit media framed it simply as an example of how mad and overbearing the EU was, which struck a cord with certain sections of the public and gently nudged them towards voting leave.
We'll 100% see similar manipulation campaigns in the run up to the 2024 election, but what's unclear just now is who the media giants/UHNWIs will support.