Why do some lecturers care about only a few selected students and make eye contact with them all the time, while marginalizing others?

Public speaking can be hard. That is a technique that is taught. Also, some faces have more of an attractiveness to the eye; this isn't necessarily about beauty. It can be about facial expressions or color, for example.

Hahahaha - marginalizing? This is a new one for me. I'm actually laughing. And if you're serious, I'm saddened.

Professors may make eye contact with students they know, students they think look interested, students who don't look threatening, etc. They may not even be making eye contact but rather staring past them to give the appearance the professor is interacting with the class.

They can sense who is really listening and who is just zoned out. The listeners naturally get more attention.

Don't jump to conclusions. It's pretty typical for people to latch onto a few friendly-looking / attentive faces during a longer presentation.