Because they're relatively easy and inexpensive and offer a truly customized look. All you need are the right tools, some instruction and a little practice. Decorating retailers can help with the first two of these requirements.
Most decorating centers offer all the necessary products to get the job done right.
Many also hold faux-finish clinics. Simply call and find out what your local retailer has to offer. It's a good idea to make a checklist of the products you'll need. Each faux-finish project has different requirements. Their proliferation in the marketplace suggests how popular specialty painting has become.