Exactly!
I had looked at taking my EMT-B in a faster format, also because of work constraints. I am a firearms instructor, and the company I worked for wanted me to get my EMT-B at the time, in case of emergency. As I didn't plan on working in EMS, I looked into the shorter schools. What I can say, quite honestly, it that the 3-4 month long class that's only a few nights a week, really includes an additional 4-6 hours of study every day and 12-24 hours of study every weekend. I don't know how you would study during a 3 week class. Even if you physically went to class and studied all night and didn't sleep for 3 weeks, there really still wouldn't be enough time to study.
If you were a medic 10 years ago and you're returning, or you are a current RN, or you have worked as a first responder for 2-3 years and "seen everything", then, obviously, you wouldn't have to study the same, because you would know a lot of stuff the rest of us didn't know going through the first time.