> I don't really understand why most if not all bottles don't use aluminum instead of plastic.

Because aluminium is a lot more expensive and difficult to manufacture at scale than plastic. It's also a lot more reusable, but reusing aluminium costs energy, and reusing glass bottles requires either energy to melt it, or an extensive recycling / reusing infrastructure (like with beer bottles over here).

Re: sparkling water, there's water fountains that can produce sparkling water on the fly now. Ask your employer to get those installed.