You maybe downloading an update file, not the full install. An update will need 10.15 installed already in order for the update to install.
In the App Store search for MacOS Catalina full install - not update.
Booting to the installer, and reinstalling macOS through that method (booting to the installer) does not erase your hard drive -- unless you choose to do that. There could have been circumstances that prevented you from doing a normal reinstall, such as an error of some kind, and you were forced to erase the drive first. That would be relatively unusual.