> I never understood why any of the smartness had to be built into the TV in the first place.

Because technology got ever more complex. A TV of yore? That thing operated on (relatively) simple physics alone, at the cost of requiring an absurd amount of broadcast infrastructure to make sure a TV signal could be received across the country.

Modern TVs however... digital modulation schemes with a lot of signal processing wizardry allow TV reception with far less broadcast towers and far better quality. You got satellites and with these a myriad of control schemes (DiSEqC, Unicable, ...). You got Pay-TV that requires decryption. You got HDMI CEC to allow your TV to remote control your DVD player.

And all of that complexity requires firmware which means it can have bugs which means you need a firmware update capability and when you're at that point you can just go and slap something Linux on it and get all the apps for streaming services.