Germany to Send Anti-Tank Weapons, 'Stinger' Missiles to Ukraine

First, even advanced MANPADS like the Stinger missile have a very constrained scope of engagement, primarily helicopters and CAS aircraft in your immediate proximity due to the limited range. Situations where you can use them don't occur all that often because adversaries have a limited number of these aircraft to cover a lot of terrain. You need to be in the middle of a ground attack or staked out at an enemy airfield, and they run patrols around airfields to make sure someone with MANPADS cannot get within practical range.

Second, many wealthier western militaries expect to be able to suppress the kinds of aircraft that MANPADS are designed for with conventional Air Force and air defense assets; MANPADS are what you use when the adversary has a dominant air attack position. If the US or NATO ever finds itself in a position where some adversary has established clear air dominance or outright superiority, a great many things have gone horribly wrong. The only time MANPADS might be useful is clandestine or covert missions where there is almost no air support at all because you aren't supposed to be there.

The US maintains a large inventory of MANPADS, but mostly for situations like the Ukraine where an ally that has no ability to establish air superiority needs to suppress the ground attack assets of someone that does. Being able to airdrop a large number of MANPADS in a very short period of time is helpful in this scenario.