Well, because mathematics can be quite all encompassing, depending upon how broadly you choose to define it. One such definition might be "the formal study of patterns": encompassing not only everything that exists, but everything that might exist and remain logically consistent. In that sense, to many mathematicians, the real world is just a special case. :-)

But in a less general sense, there's still a lot of overlap between mathematics and computer science. Mathematicians develop and prove the correctness of various algorithms: computer programmers then use those proven algorithms to solve real-world problems. Cryptography uses a lot of group theory and advanced math to develop hard to break cryto-systems; optimization problems involve a lot of calculus and algebra; neural networks involve a lot of statistics; effective compression algorithms and hashing algorithms involve probability analysis, and so forth. Even engineering problems often involve the use of partial differential equations (which again, requires calculus). Mathematics is the larger framework from which the specific tools for solving a given problem get developed.

You don't need a mathematics degree to do a "Hello, World" program. You don't need one to code up most business logic, either. You might benefit from one when doing research on just about any new thing that computers could be applied to, however.

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Ytrew Q. Uiop ( who has a B.Math, but never uses it at work)