Imo its not about raw grip strength, muscle mass OR cardio conditioning. It’s muscular endurance.
I can be super fit from months of road riding, but if I then jump on the MTB and go and do a rough trail my forearms are mangled.
On the other hand if I ride my MTB more my forearms become more resistant to the beating.
Not exactly rocket science.
OP, not the most exotic equipment ever, but I’ve found one of these helps (not the exact same one):