Because they're typically nearly irreplaceable? I also travel with delicate equipment sometimes, and my company does quite a lot, and it's usually one of a kind and either nearly impossible to replace or would cause major schedule & cost issues to some larger project. Stuff that can be carried on in a cabin generally will be, and will get its own seat. Stuff that has to be shipped gets very expensive shipping treatment, often with escort. Musicians likely can't really afford that kind of shipping.