Hi,
There are two styles of side screens, one for 1980+ which is flat in a steel frame and the early type where the clear material is bent in three places and has no frame, which is probably why glass was never considered, that and that people tend to clip the edges with their foot getting in and out so glass might not be up to the job or was considered too dangerous. Perspex would have also been cheaper than moulded glass.
Most of what has been made in the past 15 years is not perspex but Polycarbinate which is much, much stronger than perspex but also softer so it tends to scratch easier so you have to be a bit more careful when cleaning or wiping it.
Terry