Thanks to the size of the rosters and the one player per team rule, there are plenty of random players who have made an All-Star Game in their careers. Someone named Frankie Zak made it as a rookie in 1944. He played 36 games total in the rest of his career after that season.
Yet there are some genuinely good, or even great, players that somehow just never were selected. In some cases, it may be that their career was filled with decent seasons that added up to a good career, but never had one good enough to break into an All-Star team.