The quality of student knowledge

I teach at the university. I conduct classes in different courses (years of study). IT education.
I am constantly confronted with the same problem: I cannot explain anything in my own discipline, since I have to turn to the knowledge of students, which they should have received earlier, but for one reason or another they don’t. Moreover, this "earlier" is different.

You can still accept the fact that in the third year you can misunderstand something that was passed (I like that word) in the second or first. But I just can’t figure out how to get to senior university courses without having knowledge of the basic (in my opinion) subjects of the school curriculum. These subjects include mathematics (both algebra and geometry), the Russian language, and a special attitude to school computer science.

Sometimes situations come to idiotic ones. A student is sitting who has to solve a problem, and “hangs” on such things as, for example, determining the distance between two points, knowing their coordinates. Or he did the work himself, but he cannot explain what he did. I wrote the program, but I cannot tell the "logic" of the program.
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Well, if there were not very many such students, but when their number approaches half of the flow, it makes you not so much thinking, makes you sad ...

There is a hope that this is temporary, and that this is some kind of anomaly that will soon pass away. But, according to my observations, it is not necessary to wait for a quick change for the better.

I would like to ask, is it my situation alone, or is everything bad in the whole country?