I completely agree. More people who change their vote according to the performance and beliefs of what they are voting for would make the world a better place.
I suppose it's part of the legacy of us being a tribal species that we seek to define ourselves with reference to political organisations, football teams, ideologies, or other societal subcultures. And I think that's fine as long as this choice to define yourself is revocable if that organisation is changing your values rather than reflecting them.
Political parties want guaranteed support that they can count on, to shrink the pool of swing voters, and to squeeze out any smaller parties that may (given enough oxygen) develop into a threat.
If a party can portray their ‘brand’ as aspirational, while denigrating the opposition, a lot of voters won’t dig into the policy detail, or even know what the policies are, they’ll just think “this brand reflects my values” and cast their vote for them, when, if they looked at what they were actually voting for, they may find that party does not reflect their values at all, and use their vote very differently.
Blind tribalism is what enables Trump and Biden in the US and is the explicit aim of those like Lynton Crosby and his ilk.