There are proposal writers who write proposals on commission. However, for most NGOs, hiring consultants to write proposals does not amount to good use of resources. For one, it will not guarantee funding and consultants typically have a fee which they charge regardless of the success or failure of the proposal. More importantly, writing proposals is something organizations must do repeatedly and so is an essential skill every NGO relying on grants to fund projects must possess. The resources put into hiring a consultant might be better spent developing internal capacity in proposal writing.
Consultants can be great resources in certain areas. Consultants can conduct donor research, organize field research, create marketing tools etc. Consultants can also review and edit proposals written internally.