Why isn't "iar" accepted for "and"?
They are not equivalent.
You can only use şi in an enumeration (A, B şi C).
Iar implies a contrast/ difference:
Eu plec, iar tu rămâi. = I am leaving and you are staying.
Eu sunt copil, iar tu eşti adult. = I'm a child and you're an adult.
So iar is almost equivalent of 'while'. I am leaving while you are staying.
It's more like "but"