Who is they? A lot of people ask this question. Instructors hear it every now and again. Most often times, we cannot answer either. Sometimes we know where it came from, a particular event which caused the methodology change or development. For instance, North Hollywood Bank Robbery shoot out spurred the use of Ar-15s in LE. A mass shooting at Luby's in Killeen spurred concealed carry in Texas. Competition shooters developed isosceles for stability, civilians grabbed it for stability and LE grabbed it stability AND because it's more logical when using body armor.
But what about actually the mechanical methodology of using firearms? Can we answer this? Again, sometimes. But more often than not we can't. Which isn't a problem except for those people who want to know the source of everything. Also in this part of the world, there are idiots abound who repeat stuff with no basis in reality. Some have enough clout to actually get people to listen. So best case scenario is take everything with a grain of salt unless it makes sense. Most of the ideas and theories come from those who have used it in real life. We've had significant push in methodology since being involved in two wars for almost two decades and almost all of which has been urban. Which applies to private citizens and LE in a lot ways.
Lastly, does this mean it's all the gospel just because it's how it's been done? Fuck no. Don't get me wrong, some things are not up for debate. It just is what it is, no amount of changing it makes it better. Something like trigger pulling. You have to pull it straight back or try to pull it straight back. Sight alignment is what it is. But something like reloading? Sure the common method is bullets forward, index the index finger along the spine. Bring the pistol in and stay eye level while dropping the magazine and coming up with the fresh magazine all the while putting the muzzle up at a 45. But does it have to be done that way? No, not really. As long as what you're doing is safe, economical and gets it done. Then does it matter how it gets done? I personally don't think so.
But then there are those that will say "but they say...." Again, who is they?