Because most former players only have 2.5 years in the league getting paid minimum on a rookie contract. Good for $750k - 1.25 Mil over 3 years, but then things start to add up:
Taxes, buying mom a house, living in a major metropolitan area for most of the year, and being young & sorta dumb means you may not have too much other than a nice car and the house for Mom when you start your REAL career — coaching. Or selling insurance.
And most players come from poor backgrounds and have no experience or skill with handling money. You come from nothing and suddenly you have two five or ten million dollars it seems like all the money in the world. But it goes quickly when you’re supporting the extended family and your posse and others who are just hangers-on.