I can see the arguments on both sides here. Although the defences of PBTA games have been pretty robust, it really isn’t as arbitrary as it seems, there is something to the point that many other games have generative mechanics for things in the world and outside the scope of character action.
Where PBTA offers no mechanical help is where things are happening which the characters have no influence over. So yes, D&D has rules for two NPCs fighting each other, and can generate an outcome with only light input from the GM. In PBTA the GM can look at their fairly minimal stats, if they have them, but the game system doesn’t offer much help.
This is by design though, two NPCs fighting each other is rarely if ever going to be interesting. PBTA is very deliberately a player facing game system, and that comes with huge advantages in keeping the game focused on the characters.
A really interesting counter-example for me is The Black Hack. This is a really excellent rules light OSR game, along the lines of Basic D&D but with much more flexible core mechanics. The second edition contains a whole pile of resources for DMs. Monster generation systems, encounter tables, dungeon creation tools, overland encounter and terrain creation tools. It s a fantastic toolkit to help GMs generate and populate their game world and keep the game ticking along.
Looking at their games, Traveller has a whole trading system for generating worlds, their con comic factors, trade goods and prices. Uncharted worlds has a huge tool box for generating an SF universe. Many games have rules for creating or generating NPCs much as you would create a character. Pendragon has rules for generating events in the winter phase. PBTA games ofter nothing like any of that.
On the flip side, PBTA games offer tools for doing things effortlessly that other games really struggle with. The Hardholder is a great example. How many other games provide robust mechanics for creating and leading a whole community, in just a page or two of mechanics? Many of the AW playbooks would be incredibly hard to create systems for and run satisfactorily in conventional mechanics, but PBTA moves make it almost effortless.