[mmis1000]

Because it's already nearly free to add 'smart' functionality to a TV? High end boards these days are mostly arm boards at first place, and add a smart tv function to it don't really cost too much cents. (You probably just need some extra storage) Why would they want to sell a dumb TV if you can make it a smart one almost for free?

[NoGravitas]

And if you include the smart tv board, you can then lower the purchase price of the TV and make it up on ads and subscriptions. The resulting price competition on smart TVs is what makes it hard to buy dumb TVs.