I finished my maths PhD last year. The concept that PhD grads earn less than people who opted out of academia earlier makes a lot of sense to me.

PhDs grads aren't financially driven - they are mathematically driven - to the extent of thinking about problems all day. Everyone I knew used to keep a notebook and pen by the bed because waking up in the night with an idea was so common. With this much passion, the desire to leave academia for a well-paid industry job is low.