Besides the nefarious reasons they might not want to make dumb TVs, there are some decent reasons.
Most TVs include speakers. Even the non-smart ones. By adding networking and apps, that TV can be used stand-alone.
For people starting out with their first home theater setup, they can get started with just the TV. With a dumb TV, they'd need the TV, and a receiver/speakers setup or a soundbar, and something to provide content like a Blu-ray player or cable box or Roku.
By making the TV smart, the TV maker has a better chance of having people start out with a higher end stand-alone TV and then adding other components later. I presume that the TV maker would rather have the consumer spend all of their initial budget on the TV, instead of splitting it between TV, receiver, and Blu-ray player.
For people with existing home theater setups, a similar thing goes on with upgrades. For example, I have a Denon AVR-1913 receiver, a Comcast X1 DVR, and a Sony Blu-ray player, and a Samsung TV.
It was a smart TV but I never used it as anything other than a dumb monitor. I used apps, not but on the TV. The Blu-ray player had a full set of streaming apps, and so does the X1. The Denon receiver has an internet radio app, and also supports Spotify Connect, and supports Airplay. So there was never any need to use an app on the TV. (Also, when I tried it once out of curiosity I could not get ARC to work to send sound back to the Denon receiver).
But then the TV died. I ended up getting another Samsung, but this new one is 4K UHD.
None of my other components are 4K. I didn't want to upgrade the rest to 4K at that time, so for things I wanted to watch in 4K I used apps on the TV [1].
With dumb TVs, I probably would not have bought a 4K TV. I would have waited until my other components were 4K, and that could have taken quite a while. I tend to keep receivers until they die.
In summary, with smart TVs, the TV can meet the needs of a wider class of consumer. As long as they don't make it so you have to give the TV network access or use its smart features, it can meet the needs of almost every class of consumer. That's attractive to the manufacturers.
[1] ...which meant that I did need to figure out ARC. As with the old Samsung, it just did not seem to want to work. Then I noticed something in the Denon manual:
> When the ARC function is used, connect a device with a "Standard HDMI cable with Ethernet" or "High Speed HDMI cable with Ethernet" for HDMI.
I made sure to use such a cable...and it worked, which makes no sense. It would make sense if we were talking eARC, but with plain ARC--which is all my 6 or 7 years old Denon supports--it should work with any regular old HDMI cable.