While the halberd's blade offers a lot of options for hooking your opponent, it equally gives them an opportunity to hook your polearm and manipulate it themselves. The simpler spearhead is also lighter than that of a halberd, allowing for more sustain when fighting for long periods.
I would definitely prefer a halberd (or billhook) over a spear given the choice, but it's not as much of a direct upgrade as you might think.
Why would you ever use AI to do that? Plugging it into an actual inflation calculator gives $18.12, which took like 5 seconds to do.
https://data.bls.gov/cgi-bin/cpicalc.pl?cost1=5&year1=198101&year2=202409