There are three reasons:
1. they don't want,
2. they want but can't,
3. it's impossible.
There used to be a linguistic school in Vienna which tried to prove that while
speaking people just play different "language games" - each one putting his own meaning
into whatever he says. Thus in their opinion understanding can be never achieved.
But are people able to understand each other without words?
YES. When they love... and I guess you experienced it yourself.
NO language is needed then. And so many people spend their time and money studying
French, Spanish, German.... Don't they believe in love? ...smile... Or maybe they
have already found it and use their native tongue? Here please read the first lines to
see that they do not understand each other. And if they don't they do not love.
... life is full of paradoxes...
As for me I speak a few languages but...
... in my dream I want to see her deep eyes telling me what she
wouldn't confess even to herself...
(Art photography. Picture by Mark Arbeit, USA)