Change the primary email adress of a user via API

Hello,
why is it currently not possible to change an employees main email adress via API?
Our usecase? before an employees start date we put in his private email adress to let him recieve onboaridng emails before he has the company login.
now we want to automate that on day one the email is changed to the company email adress.
this is currently neither possible with workflow automations, since you can not use "scheduled attribute changes" in a workflow (its only possible to schedule them manually for each employee) and it is also not possible to do this via email.