Over the years I have read many stories about people hurting themselves, a bystander or destroying their firearm in pursuit of the absolute maximum velocity for that caliber and a particular weight bullet. If a published or slightly over, X caliber and X bullet weight at X velocity is not cutting it and you push it, why? Why not just buy a firearm in a cartridge that does that at published loads? IE, pushing a 308 to risky levels, when you could just get a 300WM?
On the flip side, I have been shooting pistols at a range and had people comment on a load that has a bit of a "bark", and asked what load I was shooting. And when I tell them it is a published but below MAX (Hodgdon, Nosler, Sierra, Speer, Hornady manual load), they ask why I load so hot for practice. Well if I wanted 380 performance out of a 9mm, or 40S&W performance from a 100mm, or 38special performance out of a 357, I would have just bought one those calibers. Also, you should practice with what you are going to use to defend yourself.
So why do we load trying to duplicate a caliber that already exists?
I reload for the savings (not $ wise but in rounds fired), which are not as great as they once were, the enjoyment of it, or for accuracy that I am not getting from shelf ammo.
There are legit reasons, your modern gun is much stronger than those that the caliber originally came in (45 Colt). Or the original chambered guns had a chamber blueprint error, hampering pressure levels, but factory loads have to be loaded to be safe in the short freebored rifles that exist (6.8 SPC I vs II). Or you are restricted by platform (AR15).