@King:
In-Universe reason: the fruits have major drawbacks that make it not worth it.
Also, there are only 3 people in the crew with no DFs, and one of them is away from them.
Yeah these are the main reasons here. Not to mention Doflamingo has the hook up with real Devil Fruits. From the flash back to present day it's clear he knows where to look.
I've also speculated that with the Smiles being unstable in some way it would actually be a great Trojan Horse to use on Kaido. If Doflamingo really wants to be King of the Pirates some day, seemingly arming an enemy up the wazoo could all just be part of the plan.
When I think about it further, I ask myself well why would Law cut off this supply if it can be used against Kaido? I assume Law knows about the draw backs of the Smiles, but if there is something else even more devastating about them(like Ceasar has a special chemical that melts the faces of all Smile users idk) then Law isn't fully aware of it.
@Shadowgreed:
You do know that he was boosting himself up, right?
Sentamaru was Vegapunks body guard, led around Pacifistas, seemed to have a thorough relationship with his "uncle" Kizaru and made the War Lord and CP9 destroying Luffy look like a joke. I would not be surprised if his Haki was just as world class as he made it out to be.