Because tf2 is smart. The game knows to enable cvar tf_grapplinghook_enabled whenever a mannpower map is enabled... however, with a custom map, the game doesn't know what to do, because the prefix is labeled: ctf
I did a bit of searching around, and I couldn't find a definitive answer as to how to to create a mannpower map. Interestingly, the first place you would think would have info on this- the Valve Developer Wiki- barely even acknowledges that Mannpower exists. The most helpful thing I could find was an entity in the TF2 .fgd file called tf_logic_mannpower with the description "Logic specifically for Mannpower Mode." You could try adding that to your map and see what happens, but I'm nowhere near an expert on this so it could do nothing. This guy over on Reddit made a short list of things that your map needs, but he also stated that there's nothing short of a point_servercommand that you can add to your map to make the power-ups and grappling hooks work. You could also try checking out the decompiled Valve Mannpower maps (such as Hellfire) and see if those contain any special entities or logic that makes the game recognize them as Mannpower maps.
This guy over on Reddit made a short list of things that your map needs, but he also stated that there's nothing short of a point_servercommand that you can add to your map to make the power-ups and grappling hooks work.
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