long story short, they did, although China and Japan didn't exist 15,000 years ago when it happened.

the americas were colonised by north asians through siberia and the land-bridge that is now the Bering straight. this is shown in genetics as well as language.

if nowhere in the new world has a large asian-descended population, then who are the Inuits and why are their eyes constructed the way that they are?

sources that aren't the smithsonian

Would you care to elaborate how it would have been just as easy or easier? It seems to me that this assumption is based on your personal opinion and not grounded in data.

It's hard to give a rigorous answer on why something DIDNT happen in history, but we can compare the differing situations of Europe and Asia in 1492. To colonize the Americas the Asian civilizations would have first had to discover the Americas. But they had no compelling reason to explore in that direction. Europeans on the other hand wanted to buy Asian trade goods like silk, spices, porcelain, etc. Columbus believed (erroneously) that by exploring westward he could find a profitable rout to India (hence "Indians"and "West Indies"). Instead he ran into the inconveniently placed American continents. Blocked from their true goal, Asia, the Europeans took the Americas as a consolation prize.

I can't speak for the Japanese, but I can give a little insight as to why the Chinese did not colonize the Americas. First, as mentioned before the Chinese would have had to discover America, or have had a reason to colonize the Americas after news arrived of a new continent. Secondly, China did not "colonize" in the western sense. Instead, Chinese dynasties preferred a tributary relationship with its subjects. For example, subject nations would send materiel, armies, and the like in exchange for protection, calenders, culture etc... Furthermore, China simply did not have the need to colonize new territories due to its tributary relationships with subject nations, and the fact that China was arguably the wealthiest and most technologically advanced nation until the mid 1800's. I hope this was a good enough answer.