This was actually mentioned previously in another thread:
The issue that came up is that, currently, the system only works from 1 to 6. This is a pretty small scale, meaning that being able to level up the stats shouldn’t be too common or easy–otherwise, what’s the point of having them? It gives more reason to use certain settlers for specific jobs and prevents a city of Omni-beings.
Granted, we need more implementation of these stats to have a greater effect on the game, but I feel that any sort of improvement to them should involve intensive resources and time and be a later-game possibility.
Perhaps random events or rare items might be one way to incorporate this idea earlier than anything player-made can be achieved, just to keep it limited.