It would be somewhat hard to make glass suits wouldn't it? Likewise a glass blade would do what to the alien?
Ceramics can be acid proof, yet you will never see ceramic body armor. Why? Because its the same stuff that makes up your toilets. Likewise, despite glass and ceramics having the ability to break and produce jagged edges, the fact that they are so brittle doesn't lend itself to a jabbing weapon under concussive force.
Silicone, and silicon are two different things. Silicone goes in a woman's boobs. Silicon is what you use to make computer chips and glass.
Maybe the Aliens knew that despite the glass appearing thin (which it may or may not have been) a headstrike aimed at gediman capable of punching through ferro's helmet and her skull didn't penetrate at all. Its headbutt did no damage either. Thus they knew the floor would be a better chance of escape. Since their own bodies consist of polarized silicon (which Ash says quite explicitly in Alien), they maybe they knew at some level they'd be better off eating through the floor, than the stuff their OWN bodies were made of.
