I’m not sure voting by default to one party is a consequence of registering with that party. Maybe some psychological effect.
I’d say the main cause is the fact there are only two parties to choose from, so unless you are a moderate (or these days right-moderate) it is just a waste of time looking at candidates and their policies beyond party affiliation.
The related issue is this: in some places in america, the primaries are run by the parties. So you can only vote for canidates (within a party) in the primaries IF you register with that party. In some races the primaries are the important part, because everyone knows which party will win the main election, so who gets the party nomination is basically the undecided part.
I’ve come to like Fast N Fitness: https://f-droid.org/packages/com.easyfitness/