To give the most simple and likely reason is just so that there are a restricted set of levels. Adding a "freeplay" mode that can generate random maps is not part of the plan because that would make the regular levels look less desirable. There is also many odd quirks available with special rules. It's not just because they did not think about generating random maps for those who finished the game. It is so that they can keep control over the game in a way that players are kept playing longer without risking burnout. They don't want to make the game become a chore but rather a daily task/quest. Spending money to read the last level faster by getting more energy and whatnot is just a plus for them. They know that sunk cost feeling will keep those kinds of players coming back. If not, then it was unlikely the infinite freeplay would.