Why Is A Subdomain Created For Every Add-on Domain?

Addon Domains create subdomains, as this is how cPanel handles the creation of Addon Domains.  An addon domain is technically a parked domain on the subdomain created when looking at its Apache VirtualHost entries.  The addons work and function like subdomains, but they appear as separate domains to all those looking in from the outside (your visitors).

Addon Domains will not have their own cPanel.  However, you're able to use redirects, Softaculous, create email accounts, subdomains, etc., for all your domains in your main cPanel account, just like you do for your main domain on the account.

For example, if you go to Softaculous and choose WordPress, it will show you a list of WordPress installations. There would also be an Install Now button. After clicking that, you'd choose the addon domain and directory you want to install just as you could for your main domain.

You can find a list of your Addon Domains in your cPanel by going to Home > Domains > Addon Domains.

For further information on how to Addon Domains within your cPanel, please review: Creating an Addon Domain