“Big” air defense system are good to destroy targets at middle or high altitude. Since both sides have such systems, the only “safe” place to fly is very low altitude (below 500 feet, or 150m) where you're practically hidden from air defenses' radar, but you end up being in the range of MANPADS (though shooting at a fighter jet with a MANPADS is way harder than doing the same on an helicopter)
It's really hard, it only works if the guy is ready to shoot when the plane comes by. The only thing that makes it a little easier than shooting down an helicopter with the same weapon is that jets fly much higher than choppers (which, according to modern doctrine are supposed to fly much lower than what Russians do: ten meters of less above the ground, hiding them as much as possible from MANPADS).
And Serbia downed a F-117. If something flies it can be downed, if something floats it be sunk and if something can be seen it can be hit.
Initially the UH-1 of Vietnam fame was a single engine helicopter because the expected service life was deemed to be too short to justify the expenses of a second one. Stuff that gets used gets damaged, and in war and combat destroyed. Par of the course.
