Because I am strikes don't benefit from anything that mentions weapon attacks. Only melee weapon attacks. Yes I understand how that can be confusing but basically arm strikes are considered a melee weapon attack but they are not considered a melee weapon which means you cannot do things like smite with them.

Conversely the monk has a bunch of mechanics that you can only use with an unarmed strike including flurry of blows, martial arts, stunning strike. This means the one cannot do any of these things with a monk weapon.

Alarm strikes only interact with other things that mentioned unarmed strikes or melee weapon attacks assuming setability doesn't have a requirement of wielding a weapon.

This level of isolationism doesn't really seem useful when there's no real reason they couldn't just be considered another type of weapon.