I have more assets than debt and I could pay off my mortgage but it's simply not the best way for me to spend my capital.
Additionally, I suspect this article is using a median value for "average income" but a mean value for "average debt." This paints a misleading picture.
Second, while it is useful to think of debts in terms of things like mortgages and auto loans, most of this global debt is government debt. Think locally - your local town may need 50 million dollars to build a new school. They aren't increasing taxes for one year to build the school they instead take on 50 million dollars of debt in the form of bonds. So even if you have no personal debt you still have tens or hundreds of thousands of dollars in debt which the government has taken out which can reasonably be repaid overtime.
The anecdote about secured mortgage debt might apply equally well to government debt. Most debt is serviceable.
In other words, $350k home = $300k mortgage + $50k in equity. You still have debt. Its just that the debt was exchanged for something. Its the same equation for credit card debt.
No one takes on debt without receiving something in return.
Or you buy a car. It quickly loses value, but you still have the debt based on the original purchase.
The formula still holds, but the "equity" part is negative, which leads many people to conclude that bankruptcy is a better option than paying for something they no longer have or is no longer worth what they owe.
Aside from perhaps the first year of owning a financed car, it is worth more than remaining principal on the loan for the majority of the loan's duration.