Many people drink soy milk specifically for its calcium content. In fact, soy milk itself is not high in calcium; the real sources of calcium are other soy products like tofu.
Soybeans themselves are not particularly high in calcium. 100 grams of dried soybeans contain only 191 milligrams of calcium, while our daily requirement is 800-1000 milligrams. This would mean eating 400 grams of dried soybeans to meet the daily need. However, when soybeans are processed into tofu, they are mixed with 7-8 times their weight in water. Surprisingly, 100 grams of this diluted tofu can contain up to 164 milligrams of calcium. This is because the nigari or gypsum used to coagulate the soy milk is rich in calcium ions. The high calcium content in soy products like tofu, tofu skin, and dried tofu is related to the amount of this coagulant used. This is also why "firm" tofu has more calcium than "soft" tofu. As it turns out, that very silken tofu, which uses a calcium-free coagulant, is very low in calcium, containing only 17 milligrams per 100 grams—one-tenth the amount of regular tofu. Therefore, any soy product that does not use a calcium-based coagulant will not be particularly high in calcium, and soy milk is a prime example.
However, soy milk is an important source of protein, especially for vegetarians. At the same time, soy milk contains phytosterols, which are excellent for countering animal cholesterol. Because the two types of sterols look very similar, they "compete for space" once inside the body. If you consume more phytosterols, they occupy the receptor sites for animal cholesterol, causing the cholesterol that leads to hardening of the arteries to decrease.