Flywheels can be a bit confusing, most stock setups use an externally balanced flywheel with a harmonic balancer. Internal balancing costs money and is not needed for normal use, it is cheaper for a mass engine manufacturer to get the weights of the internal close then throw on a flywheel and harmonic balancer to keep it nice an smooth. One of the first things any hot rodder does is to blue print and balance an engine, like I said that costs. My Lunati Pro Mod kit 383 uses a -0- balance flywheel, it is internally balanced the crank shaft by its self was $1,500. In stock engines this is not practical to balance and make them an affordable engine.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.