That's the key. Take a student who graduates from college saddled with student loans. His or her net worth is probably negative. The person lands a job that pays well, and they may think that from a pure income standpoint, they are doing well. They buy a new car, a home, and loads of those fun consumer items.
Current income is enough to pay the bills, but that's about it.
Unless they've made a concerted effort to pay extra toward the loans, they still have a negative net worth. In fact, buying a car and a house has added to that negative picture and may drag it on for years. If they aren't salting away much money in savings and investing, their overall financial health isn't as healthy as their regular income would make it appear. You don't want too much of it tied up in things that you can't easily convert to cash in the event of a financial emergency. A lot of real estate may boost your net worth, for example, but it wouldn't provide quick income if you lost your job.