To expand a bit...it's not that nearly everyone doesn't start out super excited about their subjects and motivated by advancing human knowledge. It's that shortly post-PhD, you realize that in order to continue doing this work you find exciting and meaningful, you have to get money (for supplies, equipment, office space, not to mention your own salary), and the funding system is inherently kind of broken. It's not just the disappearance of tenure-track jobs. To caricature somewhat:
1. Funding is limited - grant success rates in my field are currently 10% to 15% - and in order to have a chance you need some history of having done interesting work, the more the better.
2. To carry out more of your ideas (to maximize the chances that some of them will turn out to be very interesting, as well as the overall amount and speed of work) you typically need more manpower than just yourself. You may also need equipment and people to operate it. Since you're probably a bit short of money, you hire trainees (students or postdocs), who cost less. Now you are really feeling the funding pressure since their livelihoods, not only your own, depend on your getting enough funds.
3. Eventually those trainees graduate, or become senior. Now they need to apply for their own grants and you need to find new trainees. In other words, the process of science today inherently increases the number of scientists competing for funding, and because each scientist during a single career typically requires many trainees, this number increases exponentially. There is no way the research funding budget can increase that fast over the long term.
4. The constantly increasing funding pressure means more and more people become preoccupied with their own survival above most any other professional concern. In addition to politics and ladder climbing, it hurts the science directly: if some project's not likely to get future funding, you might feel you don't have time for it, even if that's what interests you most about your area.
It's a hard problem...my impression is the reforms needed would be so sweeping that I'm not sure anyone has a complete picture of what things to do instead, let alone implement them.