Explorers don’t go to nebulae. They were all picked over years ago. Only selfie enthusiasts go there now. Explorers chart places that seem boring to most.
Ah yes, those "true explorers"

Nebulae being all picked is a myth. Just a couple months ago, I went to the Seagull nebula to see how many undiscovered systems I could find on a quest to fully survey it. I didn't expect to find any: after all, it's only 3,750 ly away, and has an asteroid base with a shipyard even. Turns out that the final number was around thirty systems fully undiscovered. Granted, most of them were mass code B, about ten C, and one D that was empty - but as you said, chart places that seem boring to most.
Then there are the procedurally generated nebulae in areas with high star density. Just last week, I bagged a hundred or so systems from one, with an AFG filter - and it would have been more if I didn't want to go back to the bubble quick.
Still plenty for the taking, but yeah, if you want to get high mass systems around real nebulae, so, the low hanging fruit - well, you're out of luck.
Anyway, sorry for the OT, I just couldn't resist.