Problem is, there are many parts that can go into multiple slots. Think about right clicking a barrel as you describe. All exos have at least two weapons, and epic ones have up to four. How is the game supposed to know which weapon you want it to go on, and which slot out of the three you want to replace? If you want to indiscriminately equip things, there's always the auto-equip function that will fill empty slots.
Diablo and RPG style right click to equip would never work in this game simply because those games have one item type for each slot, so the game can automatically assume when you right click a helmet that you're putting it in the helmet slot. Bionic Dues doesn't follow that rule or assumption. When you click on a coolant tank it can go into any slot on your exo, so the game asks you which one. Your best bet is to just adapt your habits.
