They'd need to spend more time and resources for bots to work. It would be a nightmare for programmers to create 63 AI in a huge map, all doing their own thing. Whether it's driving a vehicle, navigating the huge terrain, and tactics real players use.
With CoD, it's easier to add bots because the scale is smaller. Less players, less destruction, less pathfinding, vehicles as killstreaks, etc.
Bots? Sure, they were good back in Unreal Tournament days but now? Unless you have no internet available and/or are a really bad player I see no use for bots. It doesn't make you improve your game in any way. Predictable A.I., stale gameplay, unsatisfying experience and not rewarding in any way. In fact, it numbs your reflexes.
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