The answer to 'why is my ridge vent missing shingles' is that the ridge vent shingles have likely become worn, damaged, or dislodged over time. This can happen due to exposure to the elements, improper installation, or general aging of the roofing materials. Missing ridge vent shingles can allow water, wind, and pests to enter the attic space, potentially leading to further damage.
To address this issue, you'll need to inspect the ridge vent and surrounding area to determine the extent of the problem. If the missing shingles are limited and the ridge vent is otherwise in good condition, you may be able to replace the individual shingles yourself. However, if the problem is more widespread or the ridge vent itself is damaged, it's best to call a professional roofing contractor to assess the situation and make the necessary repairs.