This is probably because their genes have something to do with it. It is hard work to be firm and consistent all the time. Any parent can be on the verge of losing control at times. You can tire yourself out being angry one day and give in from exhaustion the next. If you are finding it hard to keep it up, you need to look at why this is so.
Some parents are simply under too much stress.
They may be looking after two or three children on their own, or may be struggling with work or other problems. You may believe that it is cruel to have firm rules. Some parents feel that their child is particularly precious or sensitive and feel guilty about being firm. Sometimes parents disagree about rules and discipline it s not just you who has to be consistent, but any other adult who often looks after your child. It is often very helpful to talk through these issues with a sensible friend.