ClaytonCross
Kinder reader Inflection wanted
It's a case of the dreaded sacred cow, niche protection, that somehow made it past the previous editions. They wanted it so Monks, and only Monks, had exclusive rights to effectively punch people. Any possible damage boosting loophole (such as Sneak Attack) was denied by making unarmed strikes not count as a weapon. Additionally, this arbitrary designation future-proofed any later printed material that might have circumvented this ban (such as Greenflame Blade).
I completely agree this is why they did it. however, I do think if you have an open off hand and single handed weapon in your main hand, they should have the ability to use "Two-weapon fighting" and punch with the off hand. That's really my only complaint. Oddly Sneak Attack, which is the worst offender here as it "unarmed" does not have the Finesse or ranged qualifier. I think they could clear up a lot of this with just using physical melee, physical ranged, melee spell, and ranked spell. Then any ability requiring a weapon would be a "physical melee attack with a weapon" instead of "weapon melee attack" which also means unarmed strikes.