One of the reasons might be that there are no non-smart TVs with the same kind of features available. For instance, I am unsure whether there are non-smart large OLED TVs.
Also, price of a TV with "smart" features typically does not differ much from a price of a similar TV with only non-smart features. So, people buy them because they are getting something extra at little additional cost.
Of course one can create a DIY smart setup with any TV. But given that many people already have a TV with a capable hardware, it is in my opinion only natural that they want to turn it into a proper smart TV simply by changing its software, rather than by buying a smart PC, adding cables, etc.
Also, price of a TV with "smart" features typically does not differ much from a price of a similar TV with only non-smart features. So, people buy them because they are getting something extra at little additional cost.
Of course one can create a DIY smart setup with any TV. But given that many people already have a TV with a capable hardware, it is in my opinion only natural that they want to turn it into a proper smart TV simply by changing its software, rather than by buying a smart PC, adding cables, etc.