There are several reasons for this. For one, people change. While they go about the business of daily life, ambitions and goals change as interests grow and develop. This means that original goal may be reached through alternative routes or as happens in many cases, it may never be reached at all.
Most of us think of a career path as being that straight line for point A to point B. It's not. For those people that have no clear direction from high school, many will "fall" into a job. This being whatever they do that provides them with the required money to live. Some times on the job training leads to a new interest. For example, say you started as a young adult in a large department store. You did retail work for a year or two and then an internal job posting required a junior buyer to work for the company. You think, sure why not. So, you apply for the job and start into what could end up being a very lucrative career. However, it's probably not one you would have seen yourself doing when you left school.
No job is a waste of time. Every job offers skills that you can take to another job. These are called transferrable skills and can include everything from hard skills like data entry and computer programming to soft skills like communication and customer service skills.
It's important to understand and enjoy the job that you do. If you enjoy it, then you learn to do it well, making you good at it. Anyone that has a skill that they are good at becomes marketable.
The other thing to let go off is that rigid, "I am going to do this'' attitude. Becoming open to change and letting your career go off in tangents can be a good thing, particularly, if you are happy when you work on these tangents. Off beat jobs can become niche jobs with unique and highly respected job skills.
Another problem with career paths is that people are heading off in a direction that they won't know they like until they get there. Then what happens if they decide they don't like the career after all? That's when being open to understanding what you want versus what you thought you wanted comes into play. It's okay to change your mind. It's okay to change industries and it's definitely okay to take the skills that you've learned up until now and apply them to another area.
Consider for example the veterinarian assistant who ends up selling pharmaceutical products. We all know of several doctors and lawyers that have ended up using their skills to become popular writers.
One of the best times to look at your career path is when you are closing in on the end of it. Not that you shouldn't look at the beginning and the middle but by the time you reach the end of the road, you will enjoy looking back to see the various steps that led you to where you find yourself now. Then compare it to the ideas you held as a young adult. If they are the same, you are in the minority.
Most of start down a path only to find that life had other ideas for us.