A lot of the people making the suggestion are paying for a box. It might look the same as the built in Android TV a lot of the time, but you can have something that doesn’t choke loading apps as often, or a box you specifically chose for better support or more ability to modify the base Android.
(Yeah pirates are also part of the picture, but I have and use both of an nvidia shield TV and an Apple TV instead of the awful built in smart nonsense. TVs would be better without the extra overhead, but not connected to the internet plus a separate smart box is a lot better IMO.)