Because it is Railroading the Game. And Worse, it is the Jerk Players Railroading the DM.
Like really, say NPC Zom only had a dagger on Monday...then when the player characters see npc Zom on Firday he has a swrod and dagger. You'd go all crazy and say ''he can't have a sword, keep the FACTS consistent DM, he has a dagger 4EVER!".
I've never heard anyone argue that.
Facts being consistent means that if Zom had a dagger on Monday, then he had a dagger on Monday, and you don't get to say "no, actually he had a sword on Monday."
"Gosh 2D8HP, you are so very correct (and also good looking)"