Don’t know what gives a player a star on the field (maybe just an OVR threshold?) as I’ve only used bad teams so far in dynasty and have only had 1 on offense and 1 on defense (which is the minimum I’m pretty sure).
And it’s definitely not a glitch and EA specifically mentioned in a deep dive that teams can have many impact players.
I’m pretty sure that the answer to getting more impact players is simply to git gud with your recruiting and development of your players.