Then people would just use the ones with higher rate of fire

It's because of the ease of use various weapons have compared to others as well as having to consider what ttk is optimal. Primary weapons all have a ttk, with a few outliers like bows, a ttk as fast as 0.7 seconds unbuffed I think (feel free to correct me on that) to 1 full second or slower. This means a lot of archetypes are getting buffed or nerfed significantly either in damage or ROF. And of course, some weapons might already be hard enough to use as they are with their current stats due to some combination of range, stability, or AA. Not to mention how having like 70 stability on an SMG is comparable to like 40 on an AR of a similar ROF. How much ammo is left over after that optimal ttk? If my 900 RPM SMG requires a full second to kill with just crits, and it fires 15 times a second, that's almost half the mag with perfect accuracy. Why would I use a 140 HC with a 1 second ttk when I can use the infinitely easier to aim with 180 for the same ttk? Every weapon with a magazine has enough ammo to kill at least 3 people with forgiveness for 2-3 shots hitting the body instead of the head. For this to even come close to working, you would also need to buff and nerf basically every stat accordingly too, some archetypes balance said stats differently. But again, what is easier to use at what ranges? Why would I ever use a 140 HC if I can get the same results with a 180 and with less accuracy needed? 3 crits for a 1 second ttk? Or 2 crits 2 body with more manageable recoil for a 1 second ttk. Which makes more sense? Then you have to consider the outliers like Last Word for example with it's 0.6(?) ttk, then you have to consider any exotics that synergize or are meant to enhance that specific weapon type.