Because FTP is an insecure protocol that should never be used. We strongly recommend leaving FTP disabled and using SSH or SFTP instead.
FTP access can be restricted on a per-member and a per-site basis. These settings are completely independent; both the member accessing a site and the site being accessed must have FTP enabled in order for access to succeed.
This is to allow maximum access control granularity in situations where multiple members share access to multiple sites via our adjunct access feature. For example, if you don't want to allow FTP access to your site, and you want to make sure a member with adjunct access doesn't enable FTP access for themself and overrule you.
To enable FTP access for your membership, visit the Profile page and toggle that setting in the "Details" box.
To enable FTP access for a site, visit the Sites panel and select the site by its short name to view its Site Information page, where you'll see the FTP setting in the "SSH/SFTP Information" box.