Why do Chinese people have closed eye s?

There are three races of mankind; the remaining races are mixtures of the three. People of all races "can" have slanted (angled) eyes; downward or upward. It's the shape of their facial bone(s). Same with cheek bones, it will vary on the individual.

What your question concerns is the "appearance of a slanted" eye. Unless the individual has individual facial features as described in the first paragraph, your question concerns the EPICANTHUS SKIN FOLD (sometimes referred to as the MONGOLIAN FOLD). This is a skin fold covering the corner of the eye, giving it an almond shape.

Climate influences living creatures. Long legged creatures have evolved to wade thru shallow water; Birds of Prey have evolved extra long & powerful talons (claws). It's theorized that the large Scandinavian people are bigger than their normal European counterpart because their larger bodies are necessary to house larger lungs which are needed to breath the THIN AIR which dominates the high mountainous region in which they live. The current theory exists, that the "epicanthus skin" protects (or was originally designed to protect) the eyes of people living in the extreme cold & hot (which may include ultraviolet rays) climates of Siberia; which is the original home of the Mongolian race.