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Why? Because that is the way King run this game. Check out those videos on YouTube of levels 15,000 +. Where are the "no boosters" videos?
Guaranteed lucky boards that will help you finish a level without boosters stop at about level 10,000. As you progress towards the end of the map levels require more party poppers and/or UFOs or gold to beat than a good player can make from events. Once you get to this stage you will either have to play at a snail's pace or spend all your saved up gold to progress.
If there was a way to pass these levels without boosters, then assuredly someone, somewhere in the world, would have posted that on YouTube.