Why?
Consider the usecase of generating a user their initial password for a service, this almost always results in the user needing to immediately reset their password after initial login, this doesn't happen if someone is generating both parts of the key pair for you.
My sysadmin doesn't need to know my password, and doesn't need to know my private key or passphrase, that would allow them to impersonate me.
*this may or may not be true depending on how Enterprise-y your workplace is.
Even as a security firm we have some exceptions in our company (disk encryption password for a physical device in the office is in a shared vault, for example), or support might do a remote control session with the user present, but it's definitely not the default setup procedure for a new employee to have shared credentials for example.
It's pretty much a necessity for any larger business that device data be protected with the user's own password and with a master password known to IT so that it can be accessed when the user's password is inevitably lost.
In principle, the sysadmin should explicitly avoid knowing any of the user's secrets, because if they do, the user can shift blame onto them: "It wasn't me, it was the admin!".
I will never generate a private key for a user and will initiate a reset process for any passwords and other personal secrets revealed to me; to do anything else would be irresponsible.