Open sourcing old stuff is a lot of effort. You need to find all people involved or know the legal status of the copyright. You need to go through all the code in case you had some properitary stuff in there which you might have paid for. And you need to do all of this next to what you normally do without a direct benefit.
I was the same. UT was so much fun - bouncing flak cannon shrapnel round corners! But my LAN group all wanted to play CS or that UT mod that was similar (Tactical Ops?).
My introduction to programming was thru a different game (Subspace VIE, a MMO on dialup back in the 90s later community-remade as Continuum) when squad had a login page at a domain. I really wondered how example.com/?login worked and that led me to where I am today.
I spent probably thousands of hours playing Infiltration back in the day. I can't imagine how much time and energy must have been poured into completely re-doing a game like that.
But it's good that the games industry caught up so the infiltration-itch can be scratched in other ways.