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What have the ammo makers got that we don't have? Why can't we replicate the predictability that the ammo makers can?
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In short: pressure testing equipment.
The factories take whatever powder and primers they have and use their pressure testing equipment to find a load that comes in under the SAAMI pressure limit when used in a tight-spec'ed SAAMI test barrel. Those barrels are designed such that they will produce pressures equal to or higher than the highest pressures that barrels sold to comsumers will produce.
You could produce exactly the same ammo ONLY IF you had EXACTLY the same powder and primers (same lot numbers, not just same names) as well as the same lot of cases and bullets.
But, you don't. That is why we need to work-up our loads. If the stack-up of differences between what we have and what the loading manuals tested goes towards high pressure on every component, then it is entirely possible to exceed SAAMI pressure limits with our handloads. There is some safety margin between the SAAMI limits and the pressure that will burst a gun. But, exceeding the SAAMI limits routinely will wear a gun much faster, and leave less than necessary margins for other factors, like high ambient temperatures, dirty bores, etc.
There are some signs of excess pressure, but those depend on what pressure is the limit for a particular cartridge. For a high pressure rife cartridge (e.g., a .270 Winchester) in a strong bolt-action rifle, there are plenty of warning signs that the pressures are higher than the SAAMI limit. But, for low pressure revolver cartridges (e.g., a .45 Colt) in a weak revolver design, there will be NO RELIABLE pressure signs until you are way over SAAMI pressures specs.
SL1