I know blunt weapons werent rare, but it feels like the standard infantry weapon has always been some sort of polearm with a sharp end like pikes, halberds, billhooks, bardiches etc. However armor has been a nuisance since the early medieval period, and my understanding is that plate armor really started to become widespread in the 1400s, yet infantry seems to have still used bladed weapons instead of long hammers etc. Is there a reason for that?
Many halberds are already at about the maximum weight you want a weapon to be at (approx 2-3kg). The hafts are thick enough to survive use, but not thicker (rectangular-octagonal, about 35x25mm, is typical). If one was making a two-handed hammer or mace with the same reach (i.e., with a haft of the same length), the haft would be the same thickness (and therefore the same weight), and thus the head would be about the same weight. This suggests that if the halberd fails to penetrate armour, it will be just as effective as a blunt weapon as that equivalent two-handed hammer/mace.
This is quite different from the comparison between sword and one-handed mace/hammer, where the two weapons are balanced quite differently - the mass of the sword is concentrated near the hilt, and the mass of the mace/hammer near the head. For both the halberd and the two-handed hammer/mace, the mass is concentrated near the head.
A blunt weapon does still provide some advantages - it is less likely to be damaged when hitting armour, and less likely to get stuck if it penetrates armour. Thus, we do see some two-handed blunt weapons, such as the pollaxe. These, as the name suggests, combine a blunt striking surface (the poll) and a blade (the axe). Pollaxes appear to be, on average, shorter and heavier than halberds, often about 3kg (the weight means that they need to be shorter to remain practical). On the battlefield, the extra reach of the halberd might often be more useful than the extra weight and not-getting-stuck and no-edge-damage of the pollaxe. In addition, many soldiers will be unarmoured, or only partly armoured. In an armoured duel, on the other hand, 100% of the opponents are fully armoured, and if your weapon gets stuck (without finishing off your opponent), you don't have your comrades at your side to help you. Thus, for such a duel, a pollaxe might be the better choice.
Halberd: https://www.metmuseum.org/art/collection/search/26192
Pollaxe: https://www.metmuseum.org/art/collection/search/26758
Pollaxe: https://www.metmuseum.org/art/collection/search/26713
As it happens, the pole can give a good bit of extra force on the hit, so a pole weapon can still be of use even in a plate-common environment. u/wotan_weevil and u/WARitter have particularly relevant comments in this thread. u/WARitter expands further on pole weapons in this answer.