It shows up because dealers don't clean their scales. Those benzos and other pills begin as powder before being pressed. Also, pretty much all black market oxys or Percocets are pressed fent these days from what I've heard. Unfortunately, it takes very little fentanyl to cause an overdose, especially in those who don't have any previous history with opioids (i.e. kids taking mdma at a music festival not expecting to stop breathing) . Cannabis laced with fentanyl is nonsense.

Edit: I'm going to copy this and post it as a root level comment so others can see.

My speculation would be that the idea was that drug dealers would lace cannabis that is non-addictive with fentanyl to create more ‘loyal’ customers.

I guess, but that seems unnecessary. You can easily get loyal customers by selling decent weed at a decent price. Or even shitty weed at a shitty price if you're in a place where it's hard to come by.