Why many distant cultures like Norses and greeks had a so similiar mitology ?

They largely don't. While we today recognize both Norse and Greek religion as pantheistic, and the individual gods as closer to human in their foibles, the similarities largely end there. When we talk about mythology, we're usually not looking at a single cohesive narrative or theology - we're looking at a sporadic array of surviving materials and narratives from different periods, often filtered through a number of translators and interpreters, glossed and organized and presented as a cohesive narrative - the very authoritative Bullfinch's Mythology, for example. The reality is usually a lot more complex - partial stories from surviving documents, names on monuments and coinage, oral traditions, etc. - so that you can end up getting many variations on the same story, as collectors of fairy tales and folk tales and folk songs discovered in the 19th century. There is no single version of "Cinderella" for example that is authoritative - just the version(s) that get published and accepted by a wider audience. So it is with Norse and Greek mythology, even the names are a matter of convention.

With Norse mythology in particular, we have to be careful because no original sources from Old Norse survive - all we know about their religion and its theology derives from much later sources, after their conversion to Christianity; the "popular" image of Norse religion was promulgated after that, and in categorical efforts to present them to the public (like Bullfinch, above, or Dungeons & Dragon's Deities & Demigods, or Marvel comics) the two pantheons tend to become more standardized and made more similar to one another.

Not to metion how both celts and chinese had similiar mytologic creatures such as dragons.

This has been touched on before, so I'm going to steal part of an earlier answer from Why are dragons a part of so many different cultures?:

It might be more accurate to ask: "Why do we perceive creatures from so many different cultures as dragons?" Despite the perception that cultures around the globe have their own dragon myths, that's a bit misleading - they have their own monster myths, which many have conflated with the European label or image of dragons, even if the origins and attributes are very dissimilar - even in Europe, you've got a wide range of difference between the Langton Worm and the legends of St. George and the Dragon. A large part of this conflation of different mythological critters can be traced back to early explorers, who tried to fit newly-encountered legends into a more familiar context, and partially due to 20th-century academics who were keen on finding universal, underlying myths - The Evolution of the Dragon (1919) by G. Elliot Smith predates Joseph Campbell's more famous Hero with a Thousand Faces, but they share a similar idea in an underlying structure or basis for form of a myth - which sort of assumes a connection even when there isn't necessarily one. The idea of a "universality" of dragons in different cultures is still pushed in in academic works like David E. Jones' An Instinct for Dragons - which was criticized for falling into pretty much the exact same trap, of assuming a universality which only really exists if you insist on calling very different creatures from very different cultures as "dragons." [...] So part of the issue is that there are a number of different creatures from different cultures which have been subsumed under the general heading "dragon" (or equivalent term) when translated into a European language context - the Aztec Quetzacoatl, the Lambton Wyrm, heraldic wyvern, the Chinese long, the Babylonian sirrush, Smok Wawelski, etc. are all sort of crowded together under the term "dragon" in English, despite having very different physical appearances, habits, supernatural correspondences, etc.