Question

Why helium is exsit as he2 but other noble gases do not show this behaviour?

Answer 1

No, helium dose not exists as He2. If you make Molecular Orbital energy level diagrams for He2 you will find that bonding and antibonding orbitals have 2 electrons each.
Now, if you want to find bond order of He2 molecule you will subtract 2 from 2 and divide the result by 2. In doing so you will get the bond order of He2 molecule equal to 0, which means that He2 molecule does not exist.

Hope this will help you.