It's historical. The first Android phones didn't do this, but also had very little internal storage indeed and were useless without an SD card. Then when manufacturers started to add more storage to the devices the only way this could be used for media was if part of it pretended to be an SD card (because many apps were hard-coded to store media and files on SD). Hence they partitioned the internal storage and called one partition /sdcard (internal storage pretending to be SD).
These days the storage is not partitioned that way(*), but in order to maintain compatibility with older apps the internal storage can still be addressed as if it were an SD card, which is what you are seeing.
(*) Except on some budget Chinese devices, which for reasons nobody understands they still partition into separate areas for apps and media. This means that people unfortunate enough to buy one of these devices usually run out of space for apps really quickly.