Likewise, I have 4 across the fediverse. Several reasons:

• the splitting of the fediverse into fiefdoms means there’s simply no way to interact with one part without having an account within the fiefdom.
• unlike a monolithic platform, the instance your account is attached to has meaning. Doesn’t look proper to do wholesome interactions with a nsfw instance name attached to your username 😜
• stability (or lack thereof). We all know what happened with vlemmy and lemmy.world…
• interface. If you want the optimum masto experience you have to have a masto account.