Because after massive shifts of the public discussion to the right, everything further to the left than “insane neocapitalistic ultra-capitalism” (also in power most of the time for decades) is now “Left”.

Or you could also rephrase the question: “Why are people not old enough to have been drowned in right-wing brain-rot for decades still sane?”

(PS: Also not a UK thing, but much more universal. Brits have the additional problem of a de facto two party system, where the former left party has moved with the general right-shift for decades and is barely center now.)

So the question is wrong already: Young Brits are not that “left” actually. The vast majority in politics is a spectrum of center right to far right, with everything else an allegedly radical left minority (funnily -or rather sadly- the opposite is true for actual right-wing extremists who are quickly incorporated and normalised by moving towards them).