Lower Control Arm Pivot Bushing

So... If the outer shell size is so critical, why does the aftermarket poly bushing suppliers tell you to slide their bushing into a 50 year old shell ? Also use the existing inner sleeve. Why not supply one that's compatable.
I'm not an expert but, wouldn't the poly bushing shrink or swell when the weather changed ? I always thought a tight hole was good. Poly bushings aren't tight, they slide right in.
I've done two rebuilds in the last couple years and I'm sticking with OEM.