I don't think it's a matter of not being finished yet. It's more likely they serve a purpose further than what has been seen thus far.
yeah i guess you are right , I guess they are planing for some thing big i assume.
but such awesome stage as tropical resort should have had a trailer
but any ways the stages aren't short.
Sweet mountain act 1 was so boring that i didn't notice it was 3:26 minutes long when i saw it , That is miss understanding from some of the fans.
just take a look at the screen shots , weird stuff going on near the goals. maybe its some big feature being in every stage we don't know about it behind the goals and its might be Buggy or being developed.
and also notice in the first screen shot , its the blue pipe we saw on sweet mountain before , its seen in the top left.