My pantry is bigger than my freezer.
I can store far more food canned than I can frozen.
Plus, just freezing food usually isn’t quite good enough. If you don’t have a blast chiller that can freeze things instantly, IQF style, then you are not going to get the same results as you see in your frozen food section.
If your question is, why do we can stuff rather than just buy it, well, that’s a whole different discussion.