The trouble with letting the cap go higher than 12 is that it probably wouldn't make the game play any better. I mean, you could increase it, but what are you achieving, exactly? What's the goal, exactly? My reaction to seeing that was also "why doesn't that scale with group size" but on reflection the answer is "because that wouldn't help anything". It's just going to cause longer periods when the DM doesn't react, doesn't spend Fear, and then longer periods when the DM does. Especially because you still hit a cap, it's just you have to start spending at 14 or 17 or whenever instead.