Because Pirates can run away
As for Beast Pirates, Wano is a closed country so there's not much info about it. Akainu fears the "samurai strenght" which is based on Ryuma's legend.
I'm not saying Navy has power to destroy all Yonkos, of course not. But it's a fact that even if marines try to capture them, pirates can just run away.
Among the current Yonkos, probably only Kaido would face them.

As for Beast Pirates, Wano is a closed country so there's not much info about it. Akainu fears the "samurai strenght" which is based on Ryuma's legend.
I'm not saying Navy has power to destroy all Yonkos, of course not. But it's a fact that even if marines try to capture them, pirates can just run away.
Among the current Yonkos, probably only Kaido would face them.
She doesn't run away. She's been there for the longest time