People about cosplay...
Let me make it simple for you.
These are some factors which make this fun-filled activity very fascinated.
1) It is Fun
We are all fed up of real life sometimes, the animated world always considered as utopia and we like to experience that Utopian life as the way it is – funny and playful. It is entertaining and enjoyable.
You can express your love for the character which fascinates you. This is the best way to enjoy the fandom of a particular character that you really like it and want to hide your personality under the charm of whatever character.
People like to be a center of attention. Getting compliment on your creativity or catching the eyes of your friends towards famous movie character is always flattering.
We all like creativity, sometimes costumes are crafted by hand. The process includes sewing, cutting, using something completely different thing for entirely different purposes make you proud of cracking the unique idea that nobody ever think of.
The only place it should really matter is in competition. Don’t enter a workmanship based contest with something that you bought or commissioned, and don’t take credit for the work of others.
That said, some contests have a category for commissioned or purchased costumes, you would be fine if you entered there.
I’m going to address this question from a different viewpoint. Why is cosplay popular amongst those that do it?
There is the factor of honoring the source material/fandom that you love. There is the creative outlet. Then there is the sharing of the work with other fans and making new connections with like-minded people. Getting complimented for your work can create a sense of accomplishment, as can having a photographer ask to take your picture. A three day event like a local comic-con or Anime/SF/Fantasy Convention can be more rewarding than a two week vacation in the tropics, to some people.
This all has a spillover effect, those that aren’t doing it, see the results and fun that those that are having and want to do it themselves next time. This keeps the world of Cosplayers growing at a currently staggering amount. I’m staff on a 2,200 person Anime Convention and we are constantly amazed at the ~60% cosplay attendance.