Humans live their lives on a daily basis, and they often ignore the passage of time. Being in a relationship with a person who will never make a commitment is not always obvious until years have passed. The partner who will never leave home may take steps to show advancement, but they are stop-gap measures to silence their partner's complaints.
The person who falls for a partner who will not commit spends much of their time making excuses to family and friends. They do not recognize they are also making excuses to themselves, and they ignore their own doubts as they do this. Many of them will push their partner to advance the relationship, and they often fail to recognize their future plans will never come to pass. They wait for years, but they will never have the life they want until they recognize this person will never change.
People who fail to commit are often comfortable with their current life, and they see little reason to change. Many of them believe they have found someone who will wait a lifetime for the commitment they promise. Rather than advance the relationship, they offer excuses for keeping the status quo. They will give a person a commitment, but push off the actual date the commitment will take place. They do this in small steps to avoid complaints or loss of their partner. Even when their partner leaves, they will simply tell themselves that person will be back.
Once a person recognizes the signs of commitment failure, they will move on and find another partner. Over time, they will eventually see that this person cannot make a commitment to any relationship. Living their life without regrets and understanding the failure was not their fault is the best way for them to live their life, and then can then enjoy a real relationship with person who is willing to build a life with them.