Lots of reasons.
Just these past couple of weeks I’ve had a sister die, been held hostage, gotten engaged, admitted to some less than savory things, captained a ship, hired on 3 new crewmembers, decided to make a life of rescuing and freeing slaves across the galaxy, had a device implanted in my arm that allows me to travel through space/time, been on two different planets, gotten a bj, rescued raped and beaten family members, worked at several different jobs, become lovers with someone I waited for years for, gone to visit my parents instantly from light years away, worked a communications console on a starship, had dinner with my son the starship captain, decided to refurbish a rustbucket of a ship, had to be taught how to poke properly several times as I kept failing the test afterwards, eaten, duct taped a slaver, slept, screwed, unclogged a toilet, bought bakery goodies from someone of a different century, worked out in the gym, danced naked on a pile of crates, supplied live ants for a picnic, made an engagement diamond from scratch, been asked to loan out my lover as a fix for sexual issues, drank romulan ale and a slew of other alcoholic beverages, and eaten lobster bisque specially made for me, among other things.
It allows me to do things I never could. I do well or badly at it at various times, but I have fun. I’ve gained some awesome friends through role play. Also fun. And that, my friend, is why the hell I do this rp thing.