He never had great homerun power but after 1971, it vanished almost completely. Still, he did manage to hit a solid .288 in 1974 before falling off the cliff offensively so it's understandable that he and the Orioles felt he could still hit for a decent average and get on base entering the 1975 season. Plenty of players from that era stayed on a few years too long but they didn't have millions of dollars sitting in bank accounts with the leverage to quit while still productive.
Good point about players of that era hanging on too long. Most still needed the money and I know Brooks who lost money due to some dishonest business partners was one of them.
Brooks was not a slugger, but one of the best clutch hitters to ever wear the Oriole uniform. He certainly had a flair for the dramatic.