The problem with that is when the fan would block the natural venting if it stops working. These devices are engineered to have the heat move the air through and out of the device, and though a fan will usually do a better job of that, it will ultimately fail. When it does, if the fan body is in the way then the intended convection is blocked and the entire unit shuts down or is damaged.
Cable modems are often placed in poorly ventilated spaces that overheat. So you can get the best of both worlds by including generally unneeded fans which enable the device to function in sub optimal conditions, but can also extend lifespan in better ventilated spaces.
The alternative of passive cooling in such spaces is unreasonably expense because only a subset of devices are subjected to such conditions.
I have thought about adding a bathroom style vent fan to the closet that houses the network gear for the house. Exhaust near the top and have a cool air intake near the floor.
Bathroom fans work too if you’re able to vent through the ceiling. I can recommend the Panasonic whisper fans (available under a variety of models with “whisper” in the name). I’m using one to exhaust the warm air from my home theater emitted by the projector. I think it’s this one: