There are only two justifiable reasons to get a Bfury on PA: either you can guarantee multi-hero cleaves (like you have Black Hole or Reverse Polarity or something... maybe Vacuum?), or because it amplifies your farm.
Thing is, the former is pretty niche, and Bfury would have an appropriately niche build rate if that was why they got it. The other reason... PA isn't fucking Anti-Mage. She's OP as fuck because she can basically automatically win the game by 30 minutes at the latest if she's played intelligently and built properly, but she
WILL lose to a lot of other carries in the late game if she actually lets them catch up with her.
You don't need a Bfury to farm up your BKB, you can farm it plenty fast by building not-shit items and taking early fights and towers and using that momentum to get your BKB. Lysah already said it above: people building Bfury on PA displays a fundamental misunderstanding of how to play that hero. It would be like a straight Bloodstone rush on Storm Spirit; sure, it handles your mana problems, but you're completely skipping the ridiculous kill potential spending that gold on Orchid Malevolence instead would give you.
That's why EG won TI4, a tournament that was largely based around very early aggression and pushing the second important heroes got their ultimates, right?
Which is why Arteezy and EE apparently don't know a fucking thing about how to play her. She's a fucking garbage tier mid, but she's sent there anyway because enemy teams never fucking punish it. Until they did, and instead of realizing that PA is best given safelane farm, they just stop playing her altogether, deciding she's a "bad" hero.
Cool, you're immune to PA (except daggers, which can crit for significant damage) for 4 seconds, but now you're taking 40% more damage
from the rest of PA's team. Ogre Magi would fucking
love for you to pop that Ghost Scepter. And rushing Ghost Scepter means those heroes aren't getting early blinkers, Mekansm, Force Staff, Eul's, etc. And it's not like you can
escape PA during this; she can still reduce you to a crawl by flipping a dagger at you, and Ghost Scepter is dispelled if you try to TP out.
So they're just idiots, then? I'm sure there are some games where PA was picked with a Magnus or Enigma on the team, but I know I've seen bits and pieces of several where it was just a random assortment of heroes with the PA, meaning they're wasting gold on an extremely inefficient item,
especially since they're already wasting 700 gold on a bottle (which means you aren't even really benefiting from bfury's regen bonuses either.)