It is a pretty simple reason: your group insurance is completely tied to your employment. Should you lose your job or want to change positions and move on from that company, you will lose your current group coverage. It is also possible that the coverage it is currently providing you is not enough for you or does not meet your specific needs, as it is a policy not geared for any one specific person.
Examine your current situation and your coverage, and then decide what amount of insurance policy would be most beneficial and appropriate for you. Things to consider are which of your needs will last and which will end. When your mortgage is paid off, you will no longer need coverage for that. The same is true for when your child or children grow up and move out to be on their own. You will no longer need to cover them. Other needs might last longer and thus require more coverage.
It is also important to consider your current employment. Do you plan on staying in your current job? And, for how long do you plan to stay there? Are you planning on making a career out of your position, or are you likely to move on from your job to a new one? These are important considerations when determining if you will need additional life insurance coverage and how much additional coverage you will need.
Just because you currently have coverage does not mean that it is adequate or that it will last for the length of time that you need that coverage. Consider other types of life insurance regardless of whether you have group insurance or not.