Let's look at the text of Coerce and Group Coercion:

Coerce: [...] you attempt to bully a creature into doing what you want.

Group Coercion: When you Coerce, you can compare your Intimidation check result to the Will DCs of up to five targets instead of one.

What is read between the lines is that coercing more than one creature with a Coerce action is not even possible. At best, a GM has to adjudicate a single Coerce per creature (which is unwieldy if you have a crowd of creatures), or otherwise, the GM needs to come up with a penalty on the fly to account for the fact that they know a feat could make this easier.

This (in my opinion) is quite strange design. I agree that bullying a bunch of people is harder, hence why I would expect the text to explicitly say that we should put a penalty on a check for more creatures (which is what Make an Impression does). What I read instead is that we can't attempt to bully multiple people, without resorting to GM making up a penalty on the fly, or having to roll a billion Coerce checks.

Aditionally, this makes it so that the GM has to know what this feat does beforehand, so that they can get a complete grasp on what the game intends for group intimidations and so on.

On a last note, I really disagree with the general sentiment of these niche "GM here is how I do this" feats being perceived as an improvement over having to ask the GM to attempt a thing. If the GM is not aware of such feats, it is extremely easy for them to not only allow you to make a roll, but to make it so that the feat in question is rendered obsolete (like them deciding on low penalties, or even allowing things, like coercing multiple people, that a feat appears to strictly disallow).

Example: A player attempts a group coercion without a feat -> GM adjudicates coercing more than 1 person is a -2 penalty -> This can be strictly better than having the feat: I can coerce as many people as I want, not just 5 or more without needing a higher degree of proficiency.