because that wouldn't be balanced in any way shape or form. go into pvp - I get two extra rockets for my boots and that's a lot of extra power.. but if it stacked on top of that, pvp would become rocket fest. in pve - no boss would take any time to kill at all because everyone would be shooting those extra rockets at it the entire time.
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If that's the case they need to give you the option to choose a different perk, that renders raid boots useless (when it comes to perks)