I'm assuming there's some skill/trick/technique required to make the seastone bullets.

And that the problem is, yeah, they could steal enough samples to reverse engineer it, or the trick/technique/skill/equipment needed, or whatever. But here's what the World Government has to consider.

If this goes wrong, there starting a fight with the only power we know about that has Seastone Bullets, a place that has a Yonko Crew backing it to boot, and that has a crazy high average power level for people in general, or so we've had suggested too us. Even if it goes right, there rolling the dice on that happening. And it's the Yonko with a death wish no less, whom I'd water is going to hesitate the least out of all of them to go have at The World Government, if for no other reason because he's hoping MAYBE they can manage a way to kill him.

And heaven help them if he were to persuade just one other Yonko to back this play as well. Then it goes form "A fight we might win but at massive cost, and were not terribly confident about winning." too "A fight we absolutely will loose.".

Given that's what there risking, and the Gorosei being conservative in there approach to ruling, I could see them deciding not to risk it. Or at least not yet.