You chose an area code for your phone number. Often people choose area codes from a home state as a bit of nostalgia or affinity, but in the end you can use any number to talk to any number, so it really does not matter much.
In Mastodon you can choose a french server if you are french and agree with generally accepted french values of moderation, or you can choose a general purpose server, or host your own.
There are a lot of LGBT servers and special interest servers with strict moderation, and general purpose servers that moderate very little.
It is also a bit like choosing what city to live in... except it is -much- easier to move if you decide you do not like the vibe and wish to live somewhere else. Mastodon has built-in user forwarding to take your follows and followers with you.
If Mastodon was fully centralized like Twitter and offered that experience, it would need an immense source of funding, and need ads, and shareholders, and would be losing money rapidly until some billionare bought it out... like Twitter.
Mastodon is built so that there is no single point of failure, but that means users have no single point of signup. It is like email, or matrix. Host your own on your own domain for a few dollars a month or join a friendly server paying the bill for you.
It is really not that hard, but people are just not used to having choices on the internet anymore.
At some point someone you know picked Twitter and you did too. Maybe find some friends or public figures you like on Mastodon and see what server they are on.
You have a good analogy attempt there, but you didn't quite nail it I think, because Outlook is also an email client.
It's more like if someone asked why they need to choose between someone@gmail.com address or someone@outlook.com, but in reality email would work just fine regardless.
You are forced to chose because there is not a central blessed server that acts as a main hub.