Is there an electronics reason why implantable RFID capsules are made of glass?

Is there an electronics reason why implantable RFID capsules are made of glass? I am curious why implantable RFID capsules are made of glass because they create a huge risk of breaking under the skin causing some serious trouble. I would guess a silver/titanium encasing will be better, and, if I'm not wrong, those metals are not magnetic, so I don't see them causing a problem with the electo-magnetic fields. I'll put the same question on a medical stack exchange, and see maybe the reason is medical not electronics related.