How do things expire once you open them/ expose them to oxygen when they clearly had to be exposed to air before being sealed?

Question

Like milk goes bad a week or two after opening it but if you don’t open it, it will stay good until the expiration date? Like yogurt, sour cream, shredded cheese. All those things. I’m confused

In: Other

Answer 1

Many things stay good way past the expiration date anyway.

Many packages are filled with inert gas like nitrogen, so there is no oxygen to spoil the food.

Many manufacturing plants are way cleaner than average homes, so less mold etc. end up in the final packaging.