Who buys desktops anymore? They can pay sales tax.
Plus, if you really wanted to tax TV's and not computer monitors, they're usually different SKU's and accept different inputs. If it has HDMI, coax, or component inputs, tax it as a TV. VGA, DVI, DisplayPort, Thunderbolt, tax it as a monitor.
About 250 million people per year apparently. And for the next decade desktops are likely to continue to be sold to the tune of 200+ million units per year even when taking into account a sales decline.
The trend has turned very sharply towards laptops over the past 10-15 years. It's a moot point in any case as you can distinguish computer displays from televisions according to input format.
