You could argue Bob didn't because even after getting brutalized by Smokin' Joe Frazier he still had a go at Ali. He did get more than he'd ever been paid before so at a guess that would have answered the "why".
True with the money thing, but there are fighters who fight for pride and prestige, not just the greedy aspect of it or just to satisfy a few blood thirsty fans. A man is someone that has the guts to admit when he is overmatched by another.
I personally don't cast it in stone. Some of these guys genuinely believe they are going to beat the bigger man. This confidence is part of what makes them great. Also picture the thousands of low level guys over the years that came in and took on far better fighters against which they had virtually no chance. If they didn't step up and basically sacrifice themselves boxing would barely exist as we know it. I'm thinking of all the fodder that packs out the records of all our favorites, Holmes, Lewis, Foreman, SRR, Charles - everyone. I can understand Bob having a go for $125 000 which was big money back then. It'd take 2 or more fights to make that at 175. Here he was performing at a ridiculously high level but in a division where the money simply wasn't there. Even Spinks excepting the Qawi unification (of which he earned almost as much as 7 1/2-1 fave over Tangstad) was earning a pittance compared to when he moved up. If you're not a heavyweight you'd better be an absolute superstar and preferably in a division or divisions with stacked matchups for you. Very rare occurrence unfortunately.