Installing ERPNext later into frappe

Why is it not possible to install erpnext onto an existing site.
I have a site with frappe installed and when I am trying to install erpnext it gives me the message:

ERPNext can only be installed on a fresh site where the setup wizard is not completed
You can reinstall this site (after saving your data) using: bench --site [sitename] reinstall

Why is this the case?
Also do I lose my data if I resintall the site?

You have to reinstall the site (it will automatically reinstall Frappe and whatever apps you had installed, but of course not ERPNext) and create fresh database (so yes you will lose whatever data you had).
After that you install ERPNext before you go to the site and login.

After having Frappe and ERPNext installed only then you may login to the site and do setup wizard (you will see there are more pages to finish).