why not is the question. its a website. why do people make blogs? why make forums? there are answers to every question...
Because they can be made into a business, there is money to be made, they dont cost a fortune to start, and people are always looking for backlinks.
because directories, especially niche directories can be a good resources and if one wants, they can also earn from them.
A well designed directory is inevitably a great resource. IMHO there is no use in replicating what somebody else has already done very well. This 'me too' is not a good long term strategy. You have to innovate. And if you build it right with great content they will come.
For similar reasons a why publisher prints a newspaper. It can provide an information resource to the reader allow a business and or other to promote a product, service and or event and in that allow a connection between both reader and the advertiser. In most cases this cretes a revenue either through a paid subscription and or through paid advertising on the site such as googles adwords etc. Develop a good newspaper and people will continue to buy it an it's circulation will increase, develop a crappy one it will be used to wrap fish n chips before dying out as a bad memory. The wheels of life continue to go around, and round, and around.
It is just another concept building to monetise if it becomes popular enough, there are directories with employees