I recently started training at a parkour gym a few months ago at age 28. And had the some of the same questions in mind. After joining a parkour gym, it became crystal clear hat the sport definitely attracts a specific demographic:

Males, mainly teenagers and adults under 25.

I wouldn't be surprised if I am older than some of the parkour coaches at the gym I go to.

I've attended [beginner] classes that have been female majority, but I noticed that a good percentage of them are college-aged or recently graduated college. The one's who are the most skilled, based on appearance, would probably classified as a "masculine women" by a most people. Obviously not all skilled women in parkour fit this description (Most of the IG "famous" parkour women I follow do not). After hanging out with 2 of them and getting to know them more, I learned that they are B-girls (break dancers), so there is a lot of overlap between the two disciplines (at least the acrobatics side of it ), which can explain why they are pretty advance.

While I think having more women in coaching positions and better marketing can help, girls in general aren't interested in sports like boys are and female interest generally decreases more and more once they become adults. Adult women tend to be more into things that are stereotypically "feminine." This can explain why things like yoga and dance/aerobic classes tend to be female-majority. These practices are artistic and highlight elegance, and they generally don't cause the body to put on a lot of muscle to make women look more "masculine" like lifting heavy weights and other strength/power based forms of physical activity.

I think changing the way girls/women view parkour needs to start with their view of sports of any kind.