Because it is believed that the universe in the past was small. At dimensions of the order of 10 m, the quantum properties of matter should play an essential role, but Einstein’s theory does not take this into account. So came to the conclusion that the future theory, which correctly describes the stages of the evolution of the Universe, must combine both the general theory of relativity and the quantum theory.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.