Banknote redesign question

If the purpose of a banknote is a store of value, why do central banks redesign their banknotes so often and demonetize their old ones? Wouldn’t a strong sense of stability be key for the health of a currency?
I don’t understand why they’re demonetized. A counterfeit note can be detected whether its old or new.

You just asked a mouthful!

In short, yes you are right about stability- that is why even during design changes, many times new banknotes retain some features from the previous series (e.g. commemorating the same person/event, keeping the note's primary colour the same, etc).

But redesigning banknotes offers tangible advantages beyond artistic expression; much like with the new polymer UK notes sometimes they are updated to keep one step ahead of counterfeiting technology, e.g. inserting watermarks, holograms, metal strips or polymer windows. Even the US dollar, having remained more or less unchanged since 1928, updated the largest denomination ($100) with security features back in 2013.

Also once new banknotes are issued the old ones don't usually get withdrawn immediately; often there is a transition period of at least several years where both new and old notes are legal tender.

Thanks for your input on this subject!