Even for something as trivial as this, this is just one of those weird cases where you look around at the world, and wonder "Why are you compelled to post that you don't understand something?"

People are making, selling, designing, buying owning glossy screens. This means your mental model is way out of alignment with reality.

But that's not a problem, there are several explanations in the comments on this very page which you can read and then you will understand why people own glossy screens.

1) That was the only option if you wanted a new MacBook Pro.

2) Even now, it's the cheaper option to purchase.

2) They make blacks look blacker and screens look shinier, which is (subjectively) desirable.

3) Because of this, they're (subjectively) nicer for looking at pictures and movies on.

4) Under many use scenarios, there are no downsides because it's pretty easy to position it so there's no glare.

5) There is no ugly bezel around the edge of the screen as there is for the matte screens so it (subjectively) looks nicer.

6) It's the sort of small thing that can be easily overlooked, or not noticed if ordering online, and someone could end up buying one without knowing in advance that the glare would annoy them, then ignoring it as a minor inconvenience.