Probably because no one is motivated enough. We already have people working on all those things, but the mentality mostly seems to be "I'll work on it when I feel like it", rather than "let's finish this game".

Fortunately, the game will be flexible enough to allow you to add your own art and music into your levels.

I have an idea. We can add easter eggs like maybe unlockable music or unlockable graphics.

what do you mean by unlockable? Is this content locked from the campaign or from the level editor, or both? I think we should first focus on seeing if we can get enough graphics and music to complete the game before adding extras. Extras won't make much of a difference in how much people like the game.

Like, if you do such and such then you get this or if you find a secret you get such and such.