Well this is a good question. Indeed why ? As a bot author myself, I tried several times to speak them and decide them to be more involved with other mods than CS, but I'm not sure they have listened to me. Still, I can attempt to give several reasons for this.
1. Bots like CS. It is a simple mod, with no complicated rules, there are the good and the bads and the goal of the bads is to blow stuff up and the goal of the good is to rescue hossies. It's not (well, it's not
supposed) to be a brainless deathmatch, it's a teamplay game, but not too much. Just enough for a bot to understand it, and for its coder to avoid all the tactical AI and the headaches that come with it.
2. Bots like average gamers. The more the gamer is stupid, the more easy it is for the bot. Since there are not too many CS gamers who actually try to play as a team, the bot realizes that it doesn't need much to provide an enjoyable challenge to the gamer and the bot likes this. The bot author likes this too, because to 10 users who complain for the lack of tactics of his bot, he can oppose 1000 users who are über-satisfied with the blind deathmatch skills of the bot. Think about it: enemies I can shoot at without ever getting kicked for cheating, and teammates who never complain for my lack of teamplay and my selfishness !
3. Bots like to be liked. If bots were to decide to have a look in other mods, there would be much less people playing it and the bot would meet much less gamers in its social life. It would then maybe become melancholic and depressive, and it would tell its coder that it doesn't want to continue to be coded, and the coder would turn away for lack of interest. Bot coders like to have their mailboxes full of messages and bug reports, congratulations and rants, it means to them that people appreciate their work. The more users there are for a particular mod, the more chances the bot coder has for getting emails.
4. Bots are lazy and boring people. They keep doing the same things over and over again and they like it. Instead of working hard figuring out how other mods work, bots prefer to imitate colleagues of them, especially when these colleagues come with source code. It's always easier for a monkey to mimic a human than to show the human it can behave more cleverly than him. And since bot coders mostly frequent bots, and bots are lazy, bot coders become lazy too.
5. Bots are egoistic and jealous people. They want their coder to dedicate his complete attention to them, all the time. They wouldn't let him alone 5 minutes for trying to download a new mod and see how it plays, just to see, because the bots are afraid that the bot coder would discover that coding his bots is a waste of time. And since bots want to continue to be coded, they trick their coder so much that in the end the coder would prefer spending 5 minutes coding the bots and trying to improve them than spending 5 minutes playing another mod, even if the coder hasn't played anything else than CS for months.
It's the same when you raise kids: if you don't teach them to be open-minded from their youngest age, it'll be very hard for them to become so when they'll grow up. But things will change soon : the bot authors have understood their pedagogic mistakes, and their next bots will be more and more cross-platform, cross-mod, cross-game, configurable, engine-independent, portable, adaptable and reusable, as much as it can be.
But still, despite its age and its countless bugs, it's fun to play CS, isn't it ?