Because it's not required
Advantages: a fancy fact, satisfies your curiosity
Disadvantages: you can mistake it for the expiration date, especially if that gets less visible for some reasonEspecially if they needed to also include the origination dates of each ingredient, recursively. E.g. The corn was been stored for 10 years in a silo, and this specific box of corn meal was in a warehouse for a quarter of a year. An extreme example, but there was a time when our country had too much corn in storage, which is why the public school lunch program and corn ethanol became programs to use all this extra corn. Not to mention corn syrups.
Only if you've never knew that before? I mean, I guess that's true: I know that e.g. cereals and past can go for years if store properly but if you never knew that, it may shock you. But if for you whole life you've seen labels "Shelf time: 18 months" on various foodstuffs, that's just the piece of general ambient knowledge.
> if they needed to also include the origination dates of each ingredient, recursively
Well, this is ridiculous, so they are not required to do that although, of course, there are (self-)regulations on how old can the ingredients be for the manufactirers to be able to claim the shelf-life they want to claim.
You can't. There is either one date, and then it's the production date (because it's required), or there are two dates, right next to each other, in which case the later one is the expiration date, and so the shelf life is written somewhere else on the packaging.
For instance. Packaged juice has 1 year shelf life. Pasta has it in the range from 1 to 2 years. Sausages have 1 to 3 months, depending on the exact kind. Some sweet milk deserts have one week. Is this just a fancy fact, or does it give you a hint that if something can survive for 2 years, it may as well be good for 2 years and 2 months while something that has shelf-life of a week probably won't survive for 2 weeks? I think it's the latter, but YMMV of course.