Many of the early blockchains were simply bitcoin forks. It is much easier to simply fork bitcoin (or another existing blockchain) than to create a new one from scratch.

Not only is there less coding effort, it also is easier to get your coin listed on an exchange that already supports the coin you're forking from, because there's less technical work for them to do to support deposits and withdrawals from the exchange.

Note that forking from an existing blockchain doesn't necessarily mean the new chain has less technical quality than one made "from-scratch", either. Indeed, in some cases the smartest thing to do can be to start with an already good chain, then make it better. Most chains, including bitcoin itself, have leveraged existing code (in bitcoin's case this includes open-source cryptographic libraries, for example).