After about 2010 companies stopped providing the server binary. Games like Modern Warfare 2, Battlefield 2, etc could be played by communities in perpetuity on private servers. If the next game (MW3, BF3) were terrible, you didn't have to buy the sequel, what you had was "good enough" and you could wait for the next version to be released in 2-3 years.

With the current "closed server" model, you can't get a copy of the server code, can't host truly private servers, and when the sequel MW4, BF4 comes out, those private servers won't survive and it forces everyone to move to the sequel regardless of the quality of the game. You can technically still hire a private server for games like BF3 (circa 2012) but very few people are going to pay the $70/month to host an official one via whatever terms EA has come up with, and you absolutely can't run it with plugins, mods, and especially custom maps or game modes, you have to play it "vanilla".

Quake 3 the server is included with the game, anyone can run it, modify it and it's very plugin friendly, which is largely why it is still around today. Closed servers you can't directly access is a deliberate decision to kill the game when the sequel is released, by not allowing users to extend what they "bought". Otherwise we would still all be playing Battlefield 3 on custom maps with CTF and 128 v 128 player servers and everything else. You can modify a handful of things on the paid private servers but it's extremely limited and there's no community feedback on any of this.

Quake 3 can also be played fully offline, for various measures of "play" and "fully".