The "know it from inside out" support, etc.
Same reason people use companies like 1and1 for webhosting or one of the companies that provide Big Blue Button instances - sure, anyone can get a VPS or a dedicated 'net connection and server machine(s) and install Linux, Apache, PHP, etc. and run a site or a mail server or BBB instance. All with Free (and free) code. But not everyone is an expert at doing it, or keepign it up and running, or configuring it just right, or integrating your authentication, or answering end-users support questions or ....
And the same reason groups use vendors to host non-free stuff even when your license allows you to download and run a local version. The college I work for did it with our last learning management system - sure, we had a local instance, but it was used for testing API code, new features, confirming bugs and bug fixes the company announced, etc. Our students and instructors didn't even know about it - they only used the vendor hosted instance.