Really? Effective Interest Rate ($$ you earn through an investment) > Cost of Capital (cost of the loan) = Profit. My cadet loan had a 15 or so month deferment; plenty of time to put that money to work.
All it takes to make money is about 5 finance equations and a few hours of research per week to find investments, such as stocks or property, that are undervalued (ie. FCF > Market Cap). Building wealth with loans is effectively the same as corporations selling bonds. After receiving your money, the company invests in a project with an NPV > 0, returns your money with interest, and pockets the remaining revenue (profit). This is the simple version of "How to Make Money with Loans," but it's basically the principles of Finance boiled down to 2-3 sentences.
Consistently investing on gut feeling ("I feel like company X has great potential") is dumb, although you may occasionally get lucky and make a decent profit. Using basic finance principles to determine the current price of the future value of free cash flow is how I invest. It's not difficult, and it's sort of fun, especially when you find a stock that that is undervalued.