The major reason would be future prices and availability. It's primarily a form of speculation for most people.
This is exactly why I keep a collection of components! Every paycheck, I buy something; a few pounds of powder, a few boxes of primers, a few bricks of .22 shells...SOMETHING!
In the past 10 years, the price of a pound of powder has gone from ~$16 to $25. A box of 1,000 Federal primers has gone from $16.99 to $35. 550 .22 shells have gone from $9.99 to $???
To me, components are much more versatile as they can be used in multiple calibers and configurations. In a pinch, you could use a small rifle primer in any handgun cartridge uses a small pistol primer. This means you could use the same primer in a from .25 ACP to .45 ACP, and of course .223 rifle rounds. Many pistol powders are also very versatile. You can use Unique in anything handgun or low noise rifle rounds.
I also cast all my own bullets and have bullet moulds and reloading die sets for every cartridge I shoot. I also know how to make black powder. Self sufficiency and less dependence on others is a good thing.