Because the body does not make it easy to keep the same muscle with less fat.

For most people, it just doesn't really matter, because their strength is so far below their peak capability it won't be hard to cut some weight while maintaining strength. The closer you get to the edge of capabilities, though, the more it will matter.

If you are outside of your noob gains period and keep up your protein intake and resistance training you will minimize your muscle loss, but you'll still see some.

Bodybuilders will even take AAS that explicitly reduce catabolism of muscle mass like Anavar and still lose some muscle on cuts.