Shaq would sometimes begin the season fat and work his way into shape during the season, so there's that element of people feeling he could/should have done more. I don't know if his body could handle being a gym rat like a Kobe, he's a huge guy and his joints might have broken down, but his diet could have been better.
People often overlook Shaq's longevity though. Shaq was an elite player for about as long as Kobe was, and Kobe is someone that everyone knows had great longevity. Kobe started being an elite player either 99/00 or 00/01 (depending on your opinion) and it went to the Achilles injury late in the 12/13 season. 13 or 14 seasons as an elite player. Shaq was already beasting his rookie year in 92/93, putting up 23/14 a game. The next season, he was putting up 29/13 a game and shooting 60% from the floor. Shaq stayed an elite player through 04/05, his first year in Miami, when he finished 2nd in MVP voting.
Shaq's peak is always talked about, for good reason, but he was a great player for a long time.