What you're missing is that there's no rhyme or reason tot he way the communities are presented when someone tries to join the fediverse, what the different standards are, how they're ordered, and so on. Of course
we know that it's not a very important choice because people can just switch to a different server later, but to the average user it feels like they're having to lock themselves into something as a condition of participation, without any real idea of what they're getting into.
Going back to the Reddit example, it's easy to browse Reddit for [whatever]; when a person decides to create and account, they become part of the u/* supercommunity, and from there can start posting/voting in the particular subreddits that interest them. The fediverse does this backwards, equivalent to making people choose a default subreddit as a condition of becoming a user. It forces people to pick a home community which will shape their whole experience of the fediverse and how they will be perceived by other members of the fediverse before they have a chance to explore the system in a noncommital way.
it is absolutely infantilizing to pretend that people are unable to choose or build the communities they want, and take some responsibility in maintaining them
It takes time for people to figure out what communities they want to inhabit within a new protocol, which is why the perception of locking them into a single originating community is an antipattern. People don't necessarily want to be judged by their originating instance, because it reduces them to a one-dimensional caricature of themselves. You could join a general-purpose instance, but there are multiple general-purpose isntances, what makes one better that or differnt from another? There's no way to tell.
Critics of the fediverse model have been pointing out this rather obvious stumbling block since it was established, and fediverse stans just keep sticking their fingers in their ears going 'la la la can't hear you.'
This 'consumer' mentality needs to die, people need to learn to be proper citizens on the internet.
It has nothing to do wish consumption vs citizenship. You're demanding people tie themselves to a point of origin as a condition of existing in the fediverse, which is one of the worst aspects of meatspace. If you join a general server and then start participating in some niche topic (idk, fursuits), people in the niche instance are likely to write you off as a tourist. Conversely, if you decide to join a furry instance but then participate in discussions about sports, a lot of your interactions are going to consist of 'go away, furry weirdo'. You could create multiple acconts for your different interests, but now you have the headache of maintaining multiple accounts.
There just isn't a good reason to lock people into a particular instance in order to sign up. It's just reproducing nationality, a concept that many of us would like to dispense with altogether.