Old habit. I'm a kid of the 1990s, and we were convinced there wasn't anything cooler than a) video games and b) replacing letters with numbers. It retrospect, we might have been a little biased.
Well, just for future reference; if you're a kid of the 90s, you're well into your 30s now.
It's weird/juvenile to be typing a phrase in a manner similar to a preteen, well after 20+ years have passed. Especially in the middle of an otherwise normal message/conversation.
Usually people do this to avoid automatic censorship systems. HN certainly has censorship in place, but I'm not aware of any that targets discussion of video games.
Why do you type it like that?