It might be some sort of survey benchmark. If there’s one on each end of the bridge, it could be for measuring alignment of the movable section against the stationary section

Making brand new ones from raw sand/ore isn’t great when you consider the need to mine and refine those into something useable. Lots of energy and effort goes into that part. The difference is that glass and aluminum are essentially infinitely recyclable, while plastic is often not. It takes way less effort and minimal input of new resources to recycle a glass bottle. Hell, with a robust bottle return system you can skip over the recycling part entirely - just send them back to the bottling facility to be cleaned and refilled.