As a celebrity, your work takes up most of your time and energy, and it may not leave you with enough time to focus on PR.
Also, PR agencies that have been in business for many years know the right contacts and can strategize better.
They will contact the right media outlets and put out the right information based on the current trends. They will also offer PR consultation on the right time to make a public appearance and the ideal way to handle media controversies.
Their approach is driven by experience and understanding of the space, whereas your responses may be emotional. It is thus better to leave the task to PR specialists and focus on what you do best.