Because music theory is not a science, but the experience summed up by musicians. The definition of science depends on the following conditions: clear knowledge (can not be ambiguous or explained at will), can stand the test, and can not contradict the known facts in any applicable scope. Music theory is obviously not a qualified science.
As one of the oldest human cultures, music was born before language, not to mention abstract mathematics. When primitive people danced and sacrificed in front of the fire, music has become an abstract way of communication for human beings. With the passage of time, music has been gradually summed up by human beings. Through the gradual development of human civilization, these "notes" have gradually developed into music theories in traditional music.
At present, the traditional academic music theory is a kind of "pseudoscience" derived from the experience of the ancients. Just because traditional music theory is not a member of the tree of modern science and technology, it is practical but unknown so, just like many early medicine.
The earliest musicians found these sounds very harmonious and pleasant to listen to, but they didn't know the principle of their harmony and pleasant to listen to, but anyway, that's how I taught my students, and you used them correctly. Therefore, this musical theory of summing up experience did not understand its principle until the maturity of modern science, such as explaining why sound is harmonious from the mathematics of modern acoustic physics.
However, it is too late. At present, the traditional conservatory of music education mostly stays on learning this theoretical knowledge of summing up experience, and pays little attention to other modern sciences related to music, such as acoustics, psychology, physics, mathematics and so on.
Back to the topic itself, the biggest difference between music theory and mathematics is inequality. Modern science behind music theory, such as acoustics and physics, can be born together with mathematics. After all, this question is like asking why there is such a big difference between traditional Chinese medicine and biology, and the problem itself is not equal. Some scientific principles of TCM may be related to biology, but the theoretical knowledge of TCM has little to do with modern biology.