They can’t waive the weight limits. Planes have very narrow design parameters around weight. If you add more weight, you need to add more fuel. And then fuel it

They can’t waive the weight limits. Planes have very narrow design parameters around weight. If you add more weight, you need to add more fuel. And then fuel itself weighs a lot, so the more fuel you bring, the more fuel you need to carry the weight of that fuel. Airlines don’t fill up the plane’s fuel tanks to the top like people do with their cars. That would greatly increase fuel costs due to the aforementioned reasons. So they try to calculate the exact amount of fuel they need for the trip and then add a safety margin. If the passengers and luggage are a lot heavier than expected it delays the flight because they need to add more fuel.