I think I read somewhere that you can have a "pinned" toot which is usually used for an introduction toot and always appears at the top of your timeline when someone clicks on your name, so my guess would be that people are creating an account specifically for a list and then putting the actual list in the introductory toot.

One possible advantage to that is you can also post additions or changes to the list to that account (in addition to changing the pinned toot) and anyone who is subscribed to the list account would see the changes.

Granted that is not an elegant way to handle it, since you'd be maintaining several accounts.