[Habibi Jones]

Alchemy is easy too, but there's a reward for it.

Cartography cooking and fishing are considerably tough skills to GM and you don't get a reward for those, I didn't design it
I'm not saying that there shouldn't be a tinkering reward I'm just saying that a possible reason for not having one could be that tinkering is actually really easy to train, easier than alchemy. Especially if you have surplus lesser poison from alchemy, then you can GM remove trap which will put you at about 80 tinkering. From there you can make bottles, spoons, whatever until GM. That's pretty easy for a craft skill The reason for the easy skillgain is probably because tinkering is kind of a secondary skill to blacksmithing.