So why not include it on your flagship? Plastic sights is like putting a budget rims on a Corvette.
I have no idea why Glock does it, I don’t work there. It could be because many folks, like me, have a very specific preference for our night sights. We’d prefer to buy the gun with sights that cost next to nothing, so that we can buy what we really want and install them ourself.
I almost always end up swapping one or both of the sights on handguns I buy. Just bought a $2k STI and am replacing the front sight.