(Why are Jews supposedly so hell-bent on destroying the white race?)

That was insulting. Leave your "supposedly" at home next time.

(Wouldn't they want to keep them around for a long time)

No, they would not. They are deluded enough to believe that they don't need us despite thousands of years of objective evidence to the contrary. They really think they can kill us all off and everything will be just fine for them.

(Also, why do they like multiculturalism so much?)

First, they are direct beneficiaries of it. Second, it weakens us.

And in answer to your title question, a White person who wants to move up in the world thinks, "How can I improve myself." A Jew thinks, "How can I destroy, cheat, or undermine those who are better than I am?" (It's really not quite that, of course, because a Jew cannot entertain the possibility that they are not already the best, and they assume that everyone who is above them cheated to get there because that is the only kind of work they themselves are willing to engage in.)