In terms of pro play, it's more dependent on the meta than anything else. Since ranged supports like Zyra, Karma, and Nami dominate the bot lane and top lane carries like Kennen, Rumble and Jayce have made a comeback, Olaf is usually one of the only tanks on the team, alongside maybe a Gnar or a Braum. In that context, the damage just isn't worth it on him if he's the only front liner.
There's a good deal of wiggle room in the subclasses; Olaf, Shyvana, and Mordekaiser are all juggernauts, but they build very differently and perform very different functions. Olaf isn't picked as a diver; he's chosen as a champion that's hard to pick off and harder to ignore. The best way for him to do that is to get as tanky as possible and just let his backline do the work.