Those aren't just a chip; they're an epoxy package with a leadframe and a chip inside it. To put just a chip on a keychain, you'd have to drill a hole through it, which is difficult because silicon is so brittle—almost like drilling a hole in glass. Then, when someone put it onto a keyring, the keyring would form a lever that applies a massive force to the edge of the brittle hole, shattering the brittle silicon. Potting the chip in acrylic resin is a much cheaper solution that works better.
