That's because the adjustable sight models were always the less desired models by L.E. and civies for EDC. Never big sellers.
A 645 with adjustable sights sounds horrific. Pic of a proper, non-horrific 645 is attached.
Plus these 2nd & 3rd Gen guns were always intended to be 'street fighters' first, not lazy Sunday afternoon target or Match guns, despite the fact that they're typically very accurate outta da box.
All my S&W autos run fixed sights. No snaggy, hedge trimmer adjustables for this dude.
Just sayin'.