Given how dangerous the sport of soccer is and how low the salary is that women players get, why do they still play it?

A lot of it is relative.  What would a female soccer player be making out in the "real world?"  Even if they did get useful degrees in college, wouldn't the opportunity to travel and play the game you love for a few more years make up for not making as much in a dull office job?

This is a common mindset in all sports. Minor league baseball and hockey is full of guys who want to "chase the dream" for a few years before going back to grad school or getting 'real' jobs.  Do you know how many fringe football players get their brains beaten in every day on the practice squad rather than give up on playing?

As for the "danger" part, I don't know of many active 20-somethings that consider the danger of their favorite activities to the point where they chose other activities...and very few people rise to the professional levels if they fear injury more than they feel passion for their game.

Sometimes its a passion for them, sometimes its better than other profession 

Soccer is hardly dangerous. Top women players in England earn up to £70k. Peanuts compared to the men but a decent salary compared to most “normal” jobs.

In the USA , they can earn about 5x as much. 

Soccer isn't that dangerous and they love the game. It's what they're good at.

It's not a job to them. 

     Dangerous?    🤣