Because that's what we have seen so far from his. His arsenal so dar revolves around Speed , Agility , Invisiblity.
We saw what his AP was On Garp.
Throw Zoro there and see the difference.
" Mihawk is weaker than Zoro " , do you feel like doing this gonna prove anything.
It doesn't take much of Reading comprehension to know Mihawk is Stronger than Zoro. So is shanks , roger , Kaidou , Bigmom , Prime beard as of now.
Luffy fought seraphims for 4 hours Couldn't do shit even after realising their weakness. Make it make sense.
Or are you implying Old Gaban > Kaidou and Luffy here?