Because it's better
Sure you can hook up a PC to a telly, but unless you like buying and then trailing extra long video cables across your lounge you have to get up out of your comfy chair to change channels*, and if the content you want to see is on your phone or fondleslab, you'll be arsing around switching cables over as well.**
Compare this with a proposed iTV, want to change channel? Voice control or app control from comfy sofa, Impromptu baby photos? AirPlay. Guest comes round with a film on their device? AirPlay. Streaming video or music from the Internet on your phone? pop that stream on the TV with AirPlay, using your fondleslab to browse the enormous film library you've acquired in Plex on your PC? you can watch it on your device or the TV with a tap.
Easy, Better, way less "hold on... I think the TV is set to the wrong input..., no wait, it's the lappy displaying on the wrong screen, just need to tweak the resolution... FFS now the aspect is wrong... " and so on.
NB, 720p projector and surround sound hooked up to my PC, not hard to appreciate the value proposition here though.
* Actually, I don't get up, I have a remote app on my device, but while sometimes it works perfectly, often it would be quicker to just go over there and do it the old fashioned way anyway.
** There's no way my wife is going to mess around with cables, so any solution that relies on them will always be my job, as is she leaves the bloody cursor on screen when watching catchup, it's poking up people's noses most of the time, drives me crazy and is probably just a little psychological warfare on her part for me making her get up to put her next show on.