Part of the textbook problem is that at many large universities, the big test of gaining full tenure for a professor is to PUBLISH. They don't give a rip about teaching -- in fact few if any college/university professors have ever been instructed in the fine art of 'teaching' their discipline. So you perform research and publish, with a little classroom time on the side, letting your grad students do the heavy lifting of paper grading and the like.
Obviously, with most professors writing, the university wants part of the profits from that effort. Note how many are published by some 'university press.'
The other killer is that text book material is dynamic -- ever changing, especially in the sciences. With so many new texts coming out every year, an oldie but goodie stands little chance of survival.
Yes, it isn't fair and often isn't ethical, but it is profitable. Students are usually strapped for cash, so the temptation to crack and share is often overwhelming, hence the DRM and time limits. Doesn't make it 'right' but that's how it is.
