When you consider the work you do to earn money/whatever else you value at a job the step increase by passing tech interviews at a big company is almost always worth way more time and/or money than many people put towards it. I think students in particular need to realize the importance of these kinds of interviews relative to their schoolwork if they're forced to make the tradeoff.
I am a small participant in the Interview Prep industry since I purchased Cracking the Coding Interview - but it's no surprise people are willing to pay for interview prep (assuming it's both quality material and the individual needs that particular kind of help) when the ROI is so high