- Making servers that are somewhat resilient to getting hacked is difficult enough if you control the servers and the code and no one can see it, but publicly available server binaries? There are more than enough eyeballs from cheaters, griefers and other abusers to discover exploits.
- If you offer server binaries to people and people get hacked by cheaters, griefers and other abusers as a result, they may hold you liable for damages
- Giving away server binaries also means giving away leverage and income. With UT99-2003-2004-3, everyone can simply set up servers and you as a publisher have no way of forcing people to pay up (or to take down central servers so that people are forced to buy the successor game).
- Giving away servers also means you give away a significant amount of brand control. Parents won't care that pedophiles or Nazis can target their children because some random server admin doesn't give a shit about moderating, they will associate it with your brand.
- In a related vein, modding is also in the crossfire. Just remember the GTA San Andreas Hot Coffee mod and the parental outrage over a decade ago - and today the influence of "concerned parents" groups has gotten even worse, not to mention legitimate concerns about people distributing pedophile or hate mods. As a publisher you can't really afford people replacing e.g. every opponent with the image of Black people.
In the end, most of the reasons boil down to people (and, at least for pedophilia and pornographic content, also governments) expecting game publishers to pick up the slack of educating people that it's not OK to distribute or spread such content, or that it's not OK to DDoS server admins because they decided to ban players for spreading hate content.