Because a file manager app asking for Full Disk Access is not suspicious, and Full Disk Access is one hell of a good way to get access to data to exfiltrate. There likely wouldn’t even be suspicion if it also asked for Internet access: if it supports connecting to network shares, you wouldn’t think twice about it having that permission.
Why is it always file manager apps?
Because a file manager app asking for Full Disk Access is not suspicious, and Full Disk Access is one hell of a good way to get access to data to exfiltrate. There likely wouldn’t even be suspicion if it also asked for Internet access: if it supports connecting to network shares, you wouldn’t think twice about it having that permission.