The problem is that someone has a full control over that device in the end (to keep this argument simple, let's ignore how apple or google fits into this pictur

The problem is that someone has a full control over that device in the end (to keep this argument simple, let's ignore how apple or google fits into this picture). And you and a company you are working for may not agree on who that admin should be. On a device I own and fully control, I would be able to create a separate user profile for work, but the company may not like how I manage the device nor can it ensure that I follow a company security policy when using my personal device. And vice versa, I won't be comfortable with creating a private profile on a company controlled device.