You Don’t Really Have 128 GB of Storage Available

You need to consider that 128 GB of storage isn’t what you’re getting, even if you buy a brand-new iPhone.
Sure, your iPhone might have 128 GB of storage capacity, but when you first boot it up, you can see that System Data and iOS are already using some of that storage.
These can collectively consume up to 20 GB of that storage, leaving you with around 100 GB for yourself. Now consider all the apps, games, and files you want to have on your iPhone, and you’ll quickly see that 128 GB is nowhere near enough.