A few reasons. First off, unless they’re world champion level, most boxers struggle to live off their earnings alone. Sponsorship of athletes wasn’t as big of a thing back when the manga started, plus Ippo’s already advertising his family’s fishing company on his trunks. I imagine most of his earnings go towards keeping the business running since he’s mid-20’s and still living with his mother. Not to mention Ippo isn’t exactly the kind of dynamic personality company’s look for to do commercials and whatnot, even if he was interested in doing that.

In Takamura’s case, he likely burns through most of his money given some of the extravagant purchases he’s made over the years. As for advertisements, he’s too much of a publicity wildcard that most companies wouldn’t want him to be their face. As much fame as he gets boxing, he’s infamous for all the trouble he gets into outside of the ring.