Short answer....cuz they don't fly unless they have to! That's why pointing dogs are used for hunting them. They'll stay hidden in the grass as long as possible. And they can run like crazy!
That said, there's a pair along our road this year and the rooster is up on the road nearly every morning. Luck for him our road is really bad right now due to frost heaves so he's pretty safe from any fast-moving vehicle. :)
Joan