For a lot of people it's tribalism. Their political party is like their football team- they would never change.
For others they have a belief of what a party stands for (regardless of whether this is actually supported by the evidence). For example some people blame all of their current financial difficulties on labour and are panicking about what happens if labour wins, even though the Tories have been in power for 14 years.
Some people wouldn't switch their vote because they support the underlying basis of that party, such as those who believe in reducing inequality generally supporting labour or those who don't care about others voting Tory.
The ones I have least respect for are those who base their reasoning on a single relatively inconsequential issue such as vat on private education, or whether labour know what a woman is (ignoring the fact that the Tories don't care about women anyway).
It beggars belief that so many people vote Tory in this country, fair enough for the independently wealthy but for anyone who relies on the NHS, state education, a functioning justice system and policing service, public transport etc, why on earth vote for a party that wants to squeeze money from every public service to enrich their own cronies?