abbodi86 has explained the reason before. All Appx packages when created, must include an unique license XML even if distributed for free.
That's why offline installers for Dell, HP, Intel or NVIDIA Appx tools always include a bundled license. If you install a newer version, it will require the matching license file for that build. When Windows installs the in-box (included) apps, there are pre-installed licenses already in the image and it can activate those apps offline.
When you download a Store app, it will never provide you an offline license. Your PC has to connect online at least
once to activate, afterwards no further network access is required. So it's impossible to run Store apps, whether integated in the image, or sideloaded -- if you're completely offline.