How? Well, that's never quite clear.
As the story progresses over four films, it becomes embarrassingly apparent that Weyland-Yutani, supposedly some kind of space-exploration company, uses all of its mainstream operations as a front for their secret master plan to collect and domesticate aliens--a project that carries a 100 percent failure rate over the two hundred years they've been trying.
"Well gentlemen, the first two centuries didn't pan out but we have big plans for the third."
And although they never see any return on their investment, their methods only become more elaborate and costly. In Aliens, for instance, they waste mind-boggling amounts of money terraforming some shithole moon just because they caught wind that there might be aliens nearby.
You can just imagine the board of directors calculating their profit margin year after year, frowning at the annual 10-billion-dollar hemorrhage that occurs every time they lose a thousand employees and a secure facility to an alien massacre they provoked. And that's before the litigation begins.
Building better worlds by slowly killing off the human race.