"Bitcoin" isn't the software, or the network, or the database. It is the consensus of every player in the ecosystem: users, miners, developers, exchanges, etc. A fork doesn't fork the consensus, its by definition a deviation from consensus. Forks don't duplicate what is valuable about bitcoin.
Except bitcoin cash seems to have maintained value.
Because there are true believers who think it will overtake bitcoin in the end. It's perceived value is derived in opposition to bitcoin, rather than duplicating bitcoin's value.
Nevertheless, there are now two competing bitcoin chains, a doubling in the number of bitcoins. And I imagine there will be more and more in the future.