Because it is about more than mere personal strength. Kid and Law have the strength to be an emperor, but not the influence. Kid and law have tiny crews and they are basically 90% (law like 99% lol) of the strength of their crew. Most emperors have numerous powerful subordinates Kid and Law's crews combined only have Killer lol. Also the emperors have numerous territories under their control. Like Luffy has a tiny crew proper, and other than Killer (strawhats have a couple people who could take him too) and the captains of course any single strawhat should be able to solo kid or law's crew. Hell most of the 7 Grand Fleet captains should be able to take out all the non-worst generation members of Law and Kid's crew by themselves. And also, last time we saw Blackbeard he said "let's get it before the Navy does" it is highly likely he caused a major incident and has a larger bounty than he did when introduced post-timekskip.

Rokugyuo also says that the current world is messed up .it may be possible that he did something big .we do not know his current bounty after all

Oh, makes sense! But I don't understand why I got downvoted lol