Then my question is why it got to be a White star, not a Japanese star.

In many Japanese TV commercials feature Gaijins, even the products they try to promote got nothing to do with them because Japanese see glamorous images of Western cultures when they see Gaijins.
On the other hand, Amricans see no glamorous images when they see Asians. So they do not use Asian actors or models even when they promote Asian products in the U.S. unless they try to make fun of Asians.

Even though I think the director of “Boss” commercials intentionally used the mannerism of Gaijins, still he thought Japanese audience was going to be intrigued and fascinated by the images of Nihon-Biiki Gaijin(Foreigners who likes Japan. And Japanese become very happy whenever they see them), especially this case is because Tommy Lee is a Hollywood star.
Even though, Tommy Lee is supposed to be an Alien in this ad, Japanese audiences are definitely seeing a Gaijin star there.
In other words, there is not much ill-intention against Gaijins behind these ads even they use the images of Gaijin’s stereotype. And when Gaijins see this, most of them feel good about their stars on Japanese TV.

Mr James’s Macdonald commercial is definitely a Hen-na-Gaijin stereotype. Then I wonder how much difference there really is between Mr James’s and Tommy Lee’s.
Well, the obvious difference is that Tommy Lee is the Hollywood star but Mr James is not, he is just a commoner. And yes, in his commercial, we may see more intentions to make fun of Gaijins, which is more offensive.
But then, still the main reason that Japanese use Gaijins in their ads is that they help to bring up the images of products. In this case, despite of his Gaijin clown persona, still Japanese are happy to see a Gaijin liking Japan, which is very different from the cases you see Asians in American media like infamous William Hung.
You do not see any Chinese person or Korena person speaking Katakoto Japanese on Japanese TV commercials to sell products except Chinese or Korean Food.

most Japanese dont know him.

It’s been framed as a “Alien is supposed to look like Tommy Lee Jones,” not “Alien is supposed to look like a White guy.” The first commercial in the series features two Japanese guys having a conversation about how odd it would be if an alien came to earth in the form of a movie star like Tommy Lee Jones.

I actually drank this stuff a lot on my recent trip to japan, tastes pretty good!