Because, for macOS Big Sur you get a full-size installer from Apple in PKG format. Double-click and open the InstallAssistant.PKG to save the “Install macOS Big Sur.app” to the Applications folder. Apart from this, some older versions of macOS are available for direct download from Apple (Sierra, EL Capitan, Yosemite) in DMG format.

1. El Capitan Reinstall

1. Post Installation is Easier: Just like windows you have to install drivers to make macOS perfect, which is pretty hard for beginners but Sierra Installer automatically do this for you by installing most drivers automatically, and Sierra Installer also installs the Bootloader for you, but you may have to install additional drivers manually in.
2. I created a USB disk with a bootable El Capitan as per this documentation. Plug-in your USB key on a turned-off computer. Start your Mac, press immediately OPTION (aka ALT) Select the Orange icon (aka a USB stick) labelled El Capitan. If it does not detect it after 10 sec and you only see your hard-drive. Try another USB port and wait 10 sec again.
3. If your computer already has OS X Yosemite installed, Unibeast will just update Yosemite to El Capitan normally, without deleting any of your apps or files. A separate hard drive: Mac OS X needs its own hard drive (a minimum of 10 GB of space is required, but at least 50 GB of space is recommended).

El Capitan Reinstall

I have a few MacPro3,1's at my school I'd like to update to El Capitan 10.11.6. Currently they're running 10.5.8. I've got really slow WiFi-internet access, but the MacPro's do not have Airport. How do I attack this?
I want to make an up-to-date USB-install stick. I know I need to get hold of El Capitan, but my personal MacBook Pro is still running Mavericks. I can only update to Sierra. So I have to find an El Capitan-running Mac, but if I manage to get hold of the El Capitan-installer, it will be tied to their Apple-ID, right? Will El Capitan install without internet access?