In both Captain Mode and Bot Team DeathMatch, the tunnel and vantage point areas are completely sealed off. In the case of Captain Mode, it force both players to face off in the center and avoid stalemate situations due to the short time limit. In the case of Bots, it's clearly for balance and AI path-finding reasons.
Additionally, on the easier difficulties of Bot Team DeathMatch, players players will spawn on the left side near the only available door. Meanwhile, on harder difficulties, they will spawn on the right side, which takes a little longer to exit the spawn area. Thus forcing players to attack bots without spawn protection.