I think the Briggs, Honda etc stuff mounted on big box store mowers and garden equipment is specifically made for HD and Lowes....... cheap. Before I bought the Scag, I only bought JD mowers from an actual JD dealer. I remember them telling (selling) me that the big box stores had these el cheapo products made just for them to target home owners for light duty use.
Commercial mowers aren't cheap. Don't remember exactly, but my 2009 48" Scag was a tad over $8k new. But since 09, I would have eaten (3) $4-5K JD riders, so I'm money ahead and I'm 100% confident that it will run for many more years trouble free. And once the engine craps, it will be cost effective to replace the motor and continue to use the same mower.