Question

wahy gases do not liquified only by compression

Answer 1

Gases liquefy when their component molecules come into contact and interact with each other; this will always happen before absolute zero because real gas particles have volume.

But an ideal gas has particles of zero volume, and no intermolecular interactions, by definition. Therefore it can't liquefy. 

Helium is actually quite close to ideality, because it's got the smallest molecules and the smallest intermolecular attractions of any gas.