Well, Laravel 8 is just out so there might be some dependencies for Laravel Nova that aren't ready for PHP 8 just yet. Give the team some time to work on the compatibility. They will tweet or send an email once this is available ;)
You can try to install it by using the --ignore-platform-reqs flag. However, it could break something in Nova.
Agreed, even when a new version of laravel comes out, there are a few tweaks Taylor makes in that first 2 weeks to a month if needed. Laravel 8 had some.