If you hire a photographer, then who owns the copyright depends on the contract you have with the photographer. You can have a contract whereby the photographer sells you the copyright along with the image.
The reason why the default owner of the copyright is the photographer is that what is being copyrighted is "creative expression". It's the same with any creative work, such as writing, painting, etc. If an author writes a novel and a publishing house publishes it, the author still owns the copyright as it their "creative expression", not the publishers. Unless, of course, they agree to sell their copyright.
If you get a new kitchen, there is no copyright involved except that whoever designed the kitchen cupboards you buy may have the copyright to that particular design. But a kitchen cupboard manufacturer would take out a patent, which is different to copyright.