My understanding is that it costs the manufacturer something like $5 difference to put what's essentially a Roku into a TV to make it "smart" (which kind of makes sense as you can buy a Roku stick for $30 retails and it has to have a case, cable, box, external hdmi connector, power etc.).

The manufacturer then sells the smart tv at a $25-50 premium over the dumb version and it's a decent profit for them.

FWIW - I was always skeptical of the onboard TV smart features, but the Samsung TV that I use a computer monitor actually is actually surprisingly decent.