Well, that's because people don't trust Oda when he tells them "X is the strongest Y" anymore. It didn't used to be like that.
I don't really care what the powerlevelers think or how Mihawk is ranked by them. People downplay Mihawk all the time so they can make their own fav sound better. It's old tea and there's no point in rehashing it. I used to have a flowchart of how the Mihawk and Vista arguments went... always so predictable.
Oda says that bounties represent threats to the WG, not strength. The WG has always thought of him as an unpredictable wild card. His threat is that he's extremely deadly and it's hard to tell what he's thinking. If his bounty is really high, it's because of that uncertainty about him.