192.168.0.0/16 range is a Private IPv4 address range as defined by RFC 1918, Address Allocation for Private Internets, but that does not mean it is defined as "internal." It means that the ISPs have agreed to not route packets with those addresses on the public Internet, but that does not mean that IPv4 distinguishes Private addresses from any other addresses. IPv4, itself, does not have the conept of Private and Public addresses.
If you look at the IANA IPv4 Special-Purpose Address Registry, it explains the special address blocks that are recognized by IPv4, and those that are not. For example, the
127.0.0.0/8 block is Reserved-by-Protocol* (recognized by IPv4), but any of the Private address blocks are not.
You seem to be forgetting that a lot of businesses have multiple routers internal to the business, and having a router demand that all Private addresses be recognized as "internal" and not routed would be a real problem. Addresses that are not reserved by the IPv4 protocol rightly need special configuration for special handling by a device because, as far as the device is concerned, they are simply IPv4 addresses.
*Reserved-by-Protocol - A boolean value indicating whether the
special-purpose address block is reserved by IP, itself. This value is
"TRUE" if the RFC that created the special-purpose address block
requires all compliant IP implementations to behave in a special way
when processing packets either to or from addresses contained by the