Alright, I'll bite.
So, basically the reason you don't see people play Olaf very often is because Olaf doesn't exactly have a good 'role.' What I mean by this is that it's difficult for Olaf to do either of the roles that most fighters look for (which is split pushing/dueling or teamfighting).
Olaf isn't good at either of those is the problem, and going full tank is a bad idea due to the nature of how his kit functions now. If you wanted to go full tank, you might as well have played Maokai, Alistar, or Jarvan because they all have the utility a tank wants since they aren't dealing much damage if they went full defensive itemization and also all have good defensive steroids to allow them to survive the frontline. Basically, they do the job exponentially better, and Olaf that's full tank doesn't really output enough meaningful damage to draw pressure. It's like full tank Riven diving, not really that scary.
Olaf isn't the best split pusher due to 2 issues: He lacks mobility and lacks burst. The reason why you see champions like Jax, Tryndamere, Fizz, Zed, etc. split push and do well is that all of them have burst and/or mobility, so that way they can get to whatever target that tries to defend and kill them fast, creating pressure instantly as a result, or they can simple run away rather easily if a team tries to collapse on them (Also due to them being able to itemize much more offensively, they can break towers faster). If Olaf tries to split, he can get stalled for a while, or can be collapsed on very easily.
Olaf does do decent damage, but the problem is that it's sustained damage, not burst (This does not mean he needs burst in his kit), and without mobility to get to his target and actually get it off, it's very difficult to actually get a kill if the enemy player can waveclear/kite. Now, you can argue that it's harder to kite Olaf, and that can be true, but due to not having a defensive steroid when he dives in, it's not too hard to kill him if the enemy team decides to focus him down/can get screwed over by a well placed flash over a wall/tower kiting.
Now we move on to teamfights for the moment. Olaf is absolutely horrible in teamfights. He provides very little utility, has next to non-existing defensive steroids (he loses his only defensive bonus using his ultimate), and has to run at his target, so it's fairly easy to see Olaf coming. Now, you can add additional mobility into his kit (ghost, ghostblade, etc.), but he still has to run through a team in order to kill his target, and as a result he will end up taking a massive amount of damage. You could make the argument that Olaf could instead peel for his team and try to dps their frontline, but once again, Olaf has no real defensive steroids, so the enemy frontline will out-tank Olaf. And also the bonus stats Olaf receives from his W is rather mediocre to be perfectly honest. I'm not saying lifesteal is a bad stat on Olaf at all, but trying to use lifesteal as an argument for being a defensive stat due to his W isn't really good enough. Lifesteal is mostly used for 2 things: Sustaining while laning/pushing, or actually healing for a large amount often. If you want to heal for a large amount, you need armor penetration (more specifically, last whisper, which isn't the most efficient item on Olaf due to one of his abilities not benefiting at all). Lifesteal tanking just doesn't work well on Olaf since he takes too long to be in melee range of squishies and takes too much damage to stay in a backline if he built items to heavily lifesteal, and that's not even including the fact that ignite/exhaust would completely nullify lifesteal. If he tries to lifesteal tank off of a frontline, he'll heal for much less and would have to soak up the damage from a backline, which he won't out-heal. Might as well play Mundo if you want to regenerate tank since it's more reliable and scales better.
Basically to sum it up, Olaf doesn't really do anything that particularly well. He can stand toe-to-toe with any fighter/tank, but most of the time, why would they?
