One central high quality HR Team can scale easily in a big org while the same amount in small companies need spend more on HR.
And its not just HR of course.
I wouldn't even trust small companies to build a modern and secure cloud product. While big companies have their own Security Teams and are able to afford a normal security audit, for a small company that means 1. much higher cost in relation and also binding of personal they might not have.
I have worked in a very small company which basically couldn't spell security. And i'm now working in a very big company. There are plenty of people who also can't do that but there are teams only for security.
I don't have to justify now that i'm doing things securely. Thats how it is. My manager will not sign the risk away; He actually can't and people are not keen of asking upper upper management for that.