I can think of a couple of reasons:
1. Access to grant funding. It's really hard to get granting organizations to take you seriously if you are applying for monies unattached.
2. Teaching load actually isn't that much of a concern for people at big research universities. One of my mentors described it like this: you have "cosmopolitan" professors and "hometown" professors. The cosmopolitans are always presenting their work, speaking at conferences where they were invited, and bringing in huge grants. They don't do much committee work, they don't have teaching loads (or, when they do, they hire someone to do the teaching for them). This leaves them plenty of time to concentrate on writing (or managing their ghostwriters aka grad students, spouses, research assistants, etc), presenting, researching, and publishing. The hometown faculty actually do all of the "dirty" work of running the university: sitting on committees, teaching, designing policy, etc.
3. They do get a cadre of grad students to help them out. Basically, they get to hand-pick a research team and work with them for 3-10 years (this happens more at large research universities and with senior faculty). These students are there to work with you because their interests and/or past work somehow connect with what you are doing (research interests, publication plans, lab work, etc).
4. Teaching a graduate-level seminar isn't like teaching an intro to technology or English 101. It's mentally tough and rigorous for everyone involved, including (and, you might argue, especially) the faculty. You have to be on your game, every week. And good faculty will use these seminars to develop and flesh out new ideas, concepts, and arguments. Like your cadre of grad students, this is another place where you get to exchange ideas on things that YOU are interested in.
That's just a couple of the reasons I think people go into it. However, I do think that opportunities to do work at this level will continue to shrink as universities are seen, more and more, as a place where you go to get trained for a job. It isn't too distant in the future that we will start seeing standardization movements in higher ed. Then you'll get everyone teaching to the test rather than their passions.