The reason retired people can’t live off of cash is simple - inflation. In many countries, governments provide and guarantee retirement. US has largely shifted to saved retirements in the form of IRA/401Ks. These are investment accounts, which means your principle is not protected, but you earn interest to, hopefully, protect you from inflation and build returns. In practice, those retired end up bearing the same market risk as normal investors, whereas a government insured pension is only as risky as the government itself (which differs by country).