Is it feasible for companies to charge game time exactly by how many hours the gamers are in the game?

For example, you buy a 30days game card, which is 30x24= 720hours, and the time doesn't count down if you're not ingame.

As gamers of course I think its a good idea, but seems to me none of the companies are willing to do this. I see it as a way to attract more crowd.

So whats holding the companies back from using this method?

Opinions:

1. Company doesn't earn if servers are down for maintenance etc