Flying is a substantially different activity. It's dangerous in its own way, often more dangerous than driving (if you're flying small planes, and probably fighter jets), but very differently so. In driving, a moment's inattention can kill, but as long as you have that taken care of, you're generally fine. In flying, things rarely go wrong with that kind of speed, but if you're not thinking far ahead, you can find yourself screwed.
A simple thought experiment: while cruising, close your eyes for ten seconds. What happens?
If you do this with a car, you're probably no longer driving when you open your eyes again. If you're on the highway and are unlucky, you never open your eyes again. If you're luckier, you've come to a stop by plowing into something. It's unlikely you're still cruising.
If you do this with an airplane, you're almost certainly still flying along fine. You may be off altitude or heading some, your airspeed may be getting into dangerous territory, but you're generally fine, and haven't suffered anything that can't be immediately fixed. There are things that can go wrong in those ten seconds to kill you, like a mid-air collision, but they're rare and not something you can avoid with confidence even with your eyes open.
This short-term ease is more than made up for with long-term planning difficulty. I can go hop in my car right now and set off for Los Angeles and be pretty sure I'll arrive safe and sound in a few days. Doing the same with an airplane (if I had one capable of making the flight, and the appropriate license) would require substantial advance planning, otherwise I could easily find myself killed by weather or lack of fuel.
In short: that pilots use HUDs is not as informative to their safety in cars as one might think.