One of the core, basic, controls in SOC 2 is:
CC6.1: The entity implements logical access security software,
infrastructure, and architectures over protected information assets to
protect them from security events to meet the entity's objectives.
There are quite a few controls in that section.
How your company chooses to exert that control is up to them. And they might have trouble explaining to internal and external assessors how using someone's existing personal account to access the company's Intellectual Property is showing due care and control over providing access.
For instance, can your company enforce and check that your personal account uses MFA when you log into GitHub? Can your company trigger password changes on your personal GitHub account if there was a suspected compromise of your account?
If I was the CISO of your company, I wouldn't allow personal accounts either. The control of your account would belong with me not with you since you are working with company assets.
However, from the clarification in the comments, they want you to set up a new personal account that they don't control either. So, it could come down to the fact that all GitHub emails about notifications, etc. go to a company email inbox and not your personal email.
As a CISO, I'd still want more control than that.