The difference is that a Roku or Fire Stick is $50 or less. If I need to upgrade my device to support a new compression standard or something, it's only $50 to replace, rather than needing to buy a whole new TV.
It's definitely more practical to buy a dumb TV and connect a streaming device, especially when smart TVs value their smart functionality at a couple hundred dollars.
I'd far rather buy a dumb OLED for $1500 and connect a $50 Fire Stick than buy a $2000 OLED with smart features, if it was an option.
Seriously, what's the benefit for having all that built into your TV?