A lot of people prefer standalone streaming boxes over integrated "smarts" because they tend to run more smoothly due to having vastly more powerful hardware and tend to receive software updates for a much longer period (see Samsung smart TVs where features stop working in 1-2 years). A dedicated streaming box also sometimes makes it easier to cut out the egregious data harvesting that smart TV makers employ to subsidize their TVs (though Roku and Amazon Fire devices are still data vacuums).

My personal setup is a 2018 Sony TV with an Apple TV 4K. The smarts on the TV aren't the worst, but the Apple TV handily beats it in fluidity, so the TV has never been connected to the internet and never will be.