It's not just them having different cooldowns. It's them having different effects, and different purposes. For example, the Rocket Launcher item you linked to is attached to armor, and is a single target ability. Xplodium Charge is an AOE ability that isn't attached to armor, and has a far shorter cooldown. Saying those are the same is like saying that the DK's Raise Dead ability is the exact same as the Book of the Dead item.
You may view it being a Trinket as no big deal, but what happens when you need to make room for a BETTER Trinket? You lose the ability forever, and considering that the damage from the Trinket doesn't scale and has nonsensical drawbacks, you'll be dumping that trinket very quickly. Again, you simply can't compare an engineering item to an ability on ANY level.
Again, we're talking about two entirely different concepts. Mining a defeated mech or golem is different than collecting scraps from mechs you built yourself. Getting a bunch of mostly useless items is different than getting an effect that enhances and restores your character immediately. One is for making items for sale. The other is to use while performing a class role.As far as scrap goes taking parts from machines you destroy to make more of your own in the exact same thing. If they added tinkers but scrap did something other then lower cool downs would they suddenly not be tinkers?