I think its because when you use your computer for work/professional purposes, the extra money is worth it because you will use your machine to earn money anyway (and the faster the machine, presumably the faster you earn it back because you save time on each project). Time is money.
Also, in professional environments you deal with tight deadlines, so saving 30% to 50% on encoding time for a video (which for larger projects could mean hours) could mean the difference between keeping your boss or clients happy and losing your job/client/money.
When you are a student, you usually do not have mission critical projects, and waiting 35 minutes for a video to encode versus 25 minutes is usually not that big a deal. Also, as a student you usually have a lot of other expenses, so saving a few hundred or thousand bucks on your equipment helps you pay your tuition, books, credit cards, etc.
Its not to say that a student couldn't use the best equipment available, cause I am sure it certainly is better/more fun/etc. to do so, but that a student doesn't NEED it as badly as a professional user who uses that equipment to actually make their living. (Much like a new driver could definitely have fun driving a BMW, but usually an entry level car will suffice for learning how to drive.)
I see where you're coming from, that is a fair point.
On the other hand though, another reason i decided to spend the extra money on a more powerful machine was to future proof myself.
For example, once i finish university (still have two years to go), the chances of me going straight into a well paid job are going to be slim, and therefore the last thing i need to be thinking about is buying a new computer because mine is no longer meeting my needs etc.
I understand that it is a lot of money for a student to pay (speaking from personal experience