Because a credit rating is based on much more than just the bills being "up to date".
For example, just one late payment can impact your score for up to two years.
The age of your accounts can affect your score. (newer accounts - lower score - older accounts - higher score)
The balance of your accounts can affect your score. If you are using more than 30% of your available credit card balances, your score will go down.
Credit inquiries. If you have applied for credit (like a loan or a new card), then those inquiries for new credit can lower your score.
Items in collections will lower your score.
Closing an older account lowers your score. (remember - the AGE of your accounts helps your score, so closing something older changes the overall average age of your accounts.)
Things like rent, electric bills, cables bills and other SERVICES are NOT reported to the credit bureaus. Paying these bills doesn't impact your score. HOWEVER, if you do happen to fail to pay these kinds of bills and they end up going to a collection agency, then it becomes a NEGATIVE report on your score.
The biggest thing that helps a credit score is TIME. The longer you continue to make on time payment on credit cards or loans, the better your score will become. Two things happen. First, you establish a long history of on time payments AND you establish account age. So, checking your score too often is not likely to show too much of a change. Checking your score once every six months or once a year might show a bigger difference because TIME is the biggest thing that helps your score when you are doing everything else right.