Probably because the owner couldn't find or afford the trim. They're actually anodised aluminium. They used to be unobtainable and only in the last year or two did they become available from places like Centreline or Classic Alfa. They arent cheap at around $300 US each and shipping would be difficult and a bit expensive due to the shape.
That raises an interesting question: Does the trim have a function beyond decoration? That is, does the gasket hold the windshield securely when the groove that is supposed to hold the trim is empty? I have heard of people stuffing a length of black 12ga wire into the groove to pad it out. Dunno if this adds anything structurally.