Because you’ve got better things to do! If you or your team are doing work that deserves an award, you shouldn’t be spending time trying to write the submissions. Not just because your time is more valuable doing what you’re good at (and paid to do) but also because you’re probably too close to what you’re doing to tell the story in the best possible way.
Expecting someone who has done something great to be able to whip up a compelling story about it is unrealistic and unfair (unless its always been part of their job description). On the other hand asking them to talk to someone else about it who can then reflect that back to them in a cohesive story will ensure they spend minimal time away from their real work and that they are prompted for all the right elements (because we do our homework on the criteria and judging process).
Investing in your submissions by bringing in a professional will not only give your entry the best chance, your people will stay focused on their work, they’ll enjoy the process and they will truly feel recognised.