Full-time workers have to balance a lot of things that include family, work, extra-curricular activities and school work. These are difficult issued for people to work around but the type of people who obtain these degrees have a level of discipline that many other student's don't have. They actually like school and learning so they are different than the average population.
Traditional schools make it difficult for students to go on and get advanced degrees. They require residencies and immersion into their school culture. Beyond the cost, students will have a hard time investing all the commuting time and campus time beyond simply the time associated with studying.
Once these graduates hit the market many move onto different careers and work. Sometimes employers reward them for their learning but in most cases they don't. They simply don't have a need for that much education. Well...they do but they don't necessarily value it the same.
While such education is important to ensure we are a scientific society that continues to develop new and exciting ideas many companies need a quick dollar ratio to justify higher costs. Thus, the challenges doctoral students face come from many different angles including the overall returns on their personal investment but those who make it through are fundamentally of higher value to learning organizations.