I think because even if the number of levels is the same, the engagement and effort is much less in Star allies. In Robobot those levels are worth more because you can't complete them just by holding right.
Planet Robobot was short as hell too. My son beat it the same day we bought it. Glad I didn’t get this one, I can wait till it’s discounted or get it used.
I need to continue playing this game. It took me over ten minutes to get through the first two levels cause I was secret hunting. That doesn't equal a short game to me.