Because mathematicians like to make up things and theories to feel important, since they are impractical people who don't do anything important in the real world.
Half-joke apart (and I studied math in college, BTW, as my major, with Sanskrit as a minor), complex numbers have many uses in the real world, in engineering and other areas.
Tons of things and phenomena in the real world are based on mathematics. Plant and leaf patterns, ocean waves, water flowing in tubes or channels, the weather, mineral and plant and animal structures, rain and snow and ice, mountains, deserts, glaciers, floods, thunder and lightning, electromagnetism, fire, etc., etc., etc. And some of those things are really based on imaginary numbers.
Mathematicians are infinitely better than statisticians, though, because the definition of a statistician is "a person who can have his head in an oven and his feet in a freezer", and say, "on the average, I am feeling quite comfortable".
Half-joke apart (and I studied math in college, BTW, as my major, with Sanskrit as a minor), complex numbers have many uses in the real world, in engineering and other areas.
See the Applications section of https://en.m.wikipedia.org/wiki/Complex_number