Maybe because the Roku interface actually works properly, whereas the apps on the smart TV are slow and buggy.

I bought a "premium" Roku box brand new for I believe $90 so I could have one with an ethernet cable. That was 3 years ago and it's currently supremely buggy. It goes green, black, crashes mid episode, if I skip a few times within a few seconds the audio might desync. It was already bad but I got a new speaker system and hooked it up to an AV receiver and it instantly got several times worse. I don't know if my experience is typical but 3 years is a disappointing lifetime for something like this IMO.

Very much this. I use the Netflix app on my smart TV as it mostly works without issue. Hulu stops with a black screen every few minutes. I haven't bothered to look to see if it has Disney+ or HBOmax apps, I just use my phone and chromecast to do anything other than Netflix.

Lots of people bought Rokus/FireTVs before they had a smart TV, and now simply prefer the interface.

I was pretty happy with my old LG TV's smart apps, but it seemed to get more and more sluggish with each update, until finally it got annoying enough that I bought a Roku.