It's not very necessary when you take the structure of the languages into account. In most cases the vowels are predictable from the grammar[0]. Your question is a bit cart-before-horse, really - they have a writing system that is perfectly adequate for what is it used for, there is no evolutionary pressure for it to adapt to.

[0] In case you are not familiar: https://en.wikipedia.org/wiki/Semitic_root#Triconsonantal_ro...