Several reasons:
The warheads are different. With a tank you want a warhead that will penetrate lots of armor, with a plane you want a warhead with lots of small dispersed fragments.
Think of shotgun, if you wanted to shoot a small bird you would not use a slug you would use birdshot. If you were shooting an elephant with a shotgun you would want a slug instead of birdshot.
Cost and effectiveness. An Anti Tank weapon does not have to be as fast as an anti air weapon. An anti air weapon needs to catch the fast plane. A fast weapons is also okay against a tank but you are spending extra for that speed that is not necessary. So I could buy more of the slower cheaper tank weapons instead.
The sensors are usually different because they look for different things. This means you need to spend more for sensors that can handle both tanks and planes when you could make a cheaper weapon that only goes after tanks and buy more of them
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