Yeah, because there’s no light at the end of the tunnel. Indefinite prison sentences means the prisoner has nothing to look forward to once they realize the parole board isn’t going to willingly let them out. At least with absurdly high sentences (like 25 years,) you at least know you’ll be released eventually. Even if the parole board doesn’t agree, you’ll still get out. But when the parole board is your only hope, that means suicide is your only other form of escape.