Now, If I throw a molotov, WHY i need to wait X turns to throw it again?
Nobody did that, so it's job for Ol' Willy to clarify the matter.
It stems to Ol' Good Fallout and Arcanum, particularly their combat systems. Fallout and Arcanum combat systems are wild - as in wild capitalism. There are overpowered skills/weapons/abilities, there are underpowered, but aside from resource management nothing stops you from abusing the most powerful tool you have. You can spam Eye Shots several times per turn every encounter, you can spam Harm - etc. While such systems have their particular charm, they are easily abused and experienced player encounters no challenge whatsoever.
Styg did the easiest thing to counter that - put cooldowns on everything. Aimed shot is basically Eye Shot from Fallout, but with cooldown. Grenades are disgustingly OP, but cooldown nerfs them a little. Cooldowns here limit the player and add more tactics to combat - you can't just spam your favourite special to dispatch of any menace (unless you play the superior BRR build, which is the closest thing you will get to vanilla Fallout skulbashery), you had to come up with the working tactic. And different types of enemies require different tactics - multiply this the number of possible builds, and here is your lauded combat complexity.
Now, what alternatives there are? There are two, and none of them will work.
You can't limit the amount of OP stuff, for Underrail is the open-ended game, with randomized loot, refreshable traders, crafting and respawning enemies. Age of Decadence has limited resources, but AoD is close-ended game and it's possible to do there. Simply not possible in Underrail.
Other way is to nerf everything to the ground. Nerf everything that has cooldown. Sadly, this will make the game boring and will completely ruin the balance. Underrail combat is quick and deadly, with people and critters dying fast; nerfing everything will turn it into a boring slog and war of attrition when everyone removes 10% of opponent HP each turn.
So, no, cooldowns in Underrail have no realistic explanation. Crafting in Underrail is quite unrealistic too. But this is the needed evil, the needed formality, without which you wouldn't get this awesome and challenging combat system.