It's a good point. In fact, you do see "model agency" too so either is acceptable.

You could look at it both ways a) just learn it as it is because you can't rely on strong rules in this area or b) consider the meaning. Doing both will help you learn better.

In terms of the meaning, "model" is the person and "modelling" is the activity.  So either option makes sense (agency for people who are models, agency for the activity of modelling).

Compare this with:

e.g. a fishing competition = a competition related to the activity of fishing

e.g. a fish pie = a pie that includes fish.

You should be able to see similar examples.

PS You are Colombian so you may prefer the US spelling - "modeling".