There are also quite a bit fewer planes in the skies and they are spread much further than cars (outside of landing/take off) and are also flown by people who went through much more rigorous and advanced training. Not to mention that most commercial pilots are former military (only real way to get enough flying hours).
You mean how everyone and their mother hacks cell phones through the air with no problem? No wait they do not
We have ways of making sure radio transmissions are secure one of those is broadcasting on a licensed band that requires specialized equipment to use.
BTW you can remote hack a car with potentially deadly results right now. Get a .22 with a scope (available at sporting goods stores), get somewhere with a good view of a major highway where people go fairly fast and start shooting out tires.