Perhaps because your TV is a cheap piece of crap?
All recent (~ 1.5 years) TVs from known "quality" brands have no problem with pc input (via DVI adapter) on HDMI ports.
Care to source your claim?
My Samsung HDTV (that's less than 1 year old) does not technically support a PC connection over HDMI either. In fact, the manual warns against it.
There are probably a few things that go into this:
1. If you have 720p HDTV, it's actual resolution is likely 1366x768. For whatever reason, most PCs and all Macs don't support this resolution natively. Additionally, if you try setting the resolution at 1280x720 you'll likely get overscan issues. The only way to truly get the correct resolution is to create a custom resolution with something like DisplayConfigX or SwitchResX. This is tedious and will take an ample amount of time as custom resolutions vary per TV.
I've read that this has something to do with 1366x768 not being divisible by 8.
If you want to avoid this issue, purchase a 1080p HDTV.
2. HDTVs are designed to scale and process images over the HDMI port. Many don't include options to turn image processing off. Therefore, if you decide to go against the manufacturer's suggestion (with something like a DVI to HDMI cable) your TV will continue to color balance, sharpen and scale the output from your computer.
For video content, this is absolutely fine. However, many manufacturers assume you're going to use your HDTV as an actual display with your PC or Mac, not just as media center screen (Front Row, Boxee, WMC, etc.)
If you use the DVI or VGA port, this usually doesn't happen. The TV will just display the input unprocessed. The computer will then have full control over all processing (on Windows, the ATI and NVIDIA control panels will allow you do color balancing, etc). This is why you see complaints from people who say their picture is "muddy" or "dark" when using the VGA port. Additionally, 1366x768 is generally not supported over VGA or DVI either (although most TVs get close, you'll likely lose a strand of vertical pixels along the edge of your screen).
There is something that can be done in software to fix issue one. For example, Windows Vista does not support 1366x768. However, if you launch Windows Media Center within Vista and select your HDTV as 720p, the screen resolution changes to be compatible (using DVI to HDMI) filling the full frame.
If you just select 1280x720 within the Windows display control panel, this does not happen (centered frame, no scaling up to 1366x768 occurs).
I don't know for sure (as I've never seen it discussed officially) but Media Center must be doing something to emulate a video signal. At that point, your HDTV knows it's OK to scale up 1366x768.
Manufacturers could easily fix issue two by allowing the ability to turn off all image processing (this is usually only available via complicated service menus). Then, consumers could decide how they want to their HDTVs to scale and process inputs.
TV supports HDMI-CEC standards. If I could hack Netflix and Hulu support into it, I'd be golden.
