A dedicated computer for this is quite expensive, and using your regular desktop is somewhat annoying in various aspects like switching screen configurations and often not possible if you don't have the PC near the TV.

Then there are the complications DRM adds, e.g. it's often not possible to get the best quality from Netflix on your PC in the browser, you only get that with the native Windows 10 app (and then not 4K unless you have some very specific GPU and drivers). The Apps on a smart TV or a FireTV/whatever don't have these restrictions.

3 C's: cost, convenience, and complexity.

First, price. I think this is the biggest factor. You need a computer (for each TV). I don't think there are too many computers (if any) in the $35-$150 range that can handle smoothly playing HD video and pushing it to an external display. Assuming the machine isn't dedicated and you already own it, it breaks point 2 below.

Second, I haven't seen a good setup for controlling a computer connected to a TV. Granted, I stopped looking a few years ago. For me (and I'd assume most people), "good setup" means ability to turn system on and control it remotely, at a minimum. I'm sure one exists, but it would definitely require even more hardware (cuts into price) and more custom setup.

Third, some people might not have the technical know-how to set up something that meets the above two things. Sure, plenty of us technical people can do it, but it's not hard to understand that we are the minority.

tl;dr there's nothing "wrong" with using a computer and an HDMI cable, but there are plenty of compelling reasons these days to use one of the ultra cheap boxes. They are basically cheap purpose-specific computers, anyway. Ultimately, different people have different needs and priorities.