Because it’s a handy name, that most people will recognize.
Influenza viruses combine and re-combine. They’ve been doing this for many,many years that we know about.
For example, one outbreak -around 2002-was pretty much traced to Chinese farmers who fed their “bird flu” infected poultry (which they had to destroy) to their pigs.
The handy, recognizable label is purposefully misleading, at best. It certainly has effectively ended any discussion of the uniqueness/speculation of source. Instead, we’ve got folks slaughtering otherwise healthy pigs and afraid to eat pork.