Because it's my hobby, since 1985.
I think that videogames is an easy hobby to perform, it's versatile cause you can get equal fun in solo against the machine or against other friends/e-friends/whatever people in PVP.
When you compare other hobbies like playing sports or music, you can play when you want, weather is not a problem (but playing RE or Eternal Darkness during rainstorms is scarrier ^^), and the most important is the fun you can get for the money you spend.
The best for me was when I played EVE online, I had so much fun for three full years, I was part of a much larger group and it was great to play with hundreds/thousands of people and building something with people I had never seen, from all over the world, whatever your face/age/sex/origins/language was... I had responsabilities in the game toward other players and it was very enjoyable. Then I thought I played a little bit too much, and I felt I had two personalities, one IG, one IRL, pressure in game was as important as in the work IRL. I realised I had crossed the yellow line and it was time for me to do something else.
Then I looked at the logs of my father who loves to play freecell on his computer, he had more than 5000 sessions, I finally understood who the real hardcore gamer was.
That's what I like in gaming : everybody can play.