You make it sound like people are sitting on piles of speed mods and decide to use something else instead. That's not the case with me. I've tried farming speed secondaries for months with almost nothing to show for it.
Speed secondaries come up extremely rarely, and when they do, the upgrades typically go to defense and speed is left unimproved. You can wind up with a final speed secondary of +10 or more once every two months or so, and I've faced people who have five or six in the +16-20 range. There's no way to compete with that.