why is movement speed from weapon stacks with anything while passive is not?

[Safiras]

Are you sure? On one of my twinks, I got +8% speed in both spots on my gun, for a total of +16%. So I did some testing with Singe, and then some with Abaddon which grants an additional 12% speed. Movement did not feel any faster with Abaddon than with Singe, so I figured the speed does not stack. There was nothing scientific about my test, just how it felt.

Ran with someone using exactly the same gear as me (Venomous speed set with Immortal daggers) and the same pet (Nekro) in the same map (Wilds). Only difference was that his daggers had a total 20% speed buff, and mine had no speed buff. He was clearly faster than me in the map. So yes, the weapon buffs stack with your existing buffs.