Why would they though?
Understand the history of this. Even a "dumb" TVs these days needs a healthy about of processing power. Decoding a 4k input video signal, decoding and applying the HDR maps, and decoding and processing all the various audio signals takes a lot of processing power. You simply cannot build a 4K TV without it also having a fairly high-end SoC - heck even a basic 4k computer monitor these days likely has a more powerful processing chip than desktop computers from 25 years ago. And if you're sticking a high-end SoC in there anyway, might as well put that SoC to further use. Making a TV smart doesn't actually increase the cost that much - a little bit more storage space, a NIC, and an OS (some of which are free!).
So comparing costs, a non-smart TV might be ~$15 cheaper than a smart TV. That's what, 3% or less of the cost of an average new TV. Totally deminimus.