Because they're supposed to be "extra" characters?
although technically they're already in the disc and just waiting to be unlocked.
Since Tekken 6, there's no more unlockable character concept since they think the concept is old. That's why all of the characters were already there from the start. The unlockables for the game was the customization items and endings. This concept then applied to Tekken TAG 2 too.
It's marketing strategy, I believe... probably wanted more people into preordering the game. Also useful in keeping the hype after release, IMO. That way people will feel there are more things to expect in the future.
Don't worry about those things. We just need to enjoy the game while waiting for the extras to be unlocked
they're free after all.
If you want to unlock things, you could play arcade mode, ghost mode (I recommend this one), and also clear the fight lab (try to get S ranks on all chapters). They'll unlock customization parts and also ending movies. All of them takes a while to complete, so I believe you won't be bored
