Because I haven't found an HTPC that does standard TV as well as a dedicated STB (speed of channel change, integration of EPG and recording, deinterlacing support etc...) nor have I found a dedicated set-top box that accesses the internet as well as an HTPC. If all you want to do is access iPlayer, then a set-top box is ideal (although I don't think any STB accesses other catchup services). Beyond that, they just don't cut it at the moment.
Having said that, some of the higher end TVs will stream media from a home network, but again, compatibility is the issue. When I've tried a couple of rips I've done here, I think only a Samsung I tried played them OK. The current gen Panasonics I tried just refused to play them. Can't recall if I tried LG but I'd be concerned what was and wasn't supported for the future - I wouldn't want to pay out for a completely integrated TV only to find it doesn't play something I was expecting it to. It's all about research and what other media you have and how it is stored. Some will also record off air and might suffice for what is needed. Again, it's all about personal requirements.
BTW: I'm to the far right when it comes to geekyness... I have 3 xbmc based systems - 2 with tuner cards in running MythTV backend - and a central server with ALL my media on. All the TVs also have Freeview or Freesat tuners as either integrated or separate boxes.