They call that a "carry trade", and they call it that because the people who do them tend to get carried out backward.
Seriously though: "borrow to make high-yielding investments" is a bet that the assets won't go bad, or drop in price.
If you're not leveraged, a 20% drawdown in your "income fund" is totally survivable. If you're 1x leveraged (invest $1mio, borrow $1mio, buy $2mio of assets), then a 20% drawdown becomes a 40% drawdown, and starts to look quite hairy. 1x leverage to buy stocks is essentially a very hefty bet that 1998 or 2008 or 1987 isn't going to recur, ever.
If you're 2x leveraged, you only need a 33% drawdown to completely bankrupt you.
And prime brokers usually don't let you do this any more, but if you're 25x leveraged like Long-Term Capital Management was at its peak, well, then, hold onto your hats ladies and gentlemen 'cause this is gonna be a wild ride.
Separately, like MikeDirnt said, it's also a bet that your funding costs won't go up and the yield from your investment won't go down. Carry trades tend to like funding short and investing long, so if yield curves flatten (like they're doing now!), the cost of funding is going to converge with the yield on the investment, and you might end up paying hefty interest.