The reason that planes and airlines do not allow liquids onboard the airplane is that the liquid could be a liquid explosive.
There is the potential for explosives to be made from liquids and someone who brings on liquids could be bringing on explosives and then you would not know about it until the explosion went off.
That's also why the airlines want ice and water to be completely frozen because the explosive liquid would not freeze.
So if you bring liquids on the plane you can usually do so as long as the liquid is completely frozen and not thawed out.
It's basically a safety reason why liquids are not allowed on planes.
The rationale being that terrorists could bring liquid explosives, or components to make explosives, through security in water bottles.