Actually the "we can just build this ourselves" is a hard thing to do. Most modern companies do not want to spend money on things like this which they can buy off the shelf. Maintaining a thing like your CI servers need people resources and such resources cost way more than what a service provider will charge. I am sure in some situations it will make sense but for the most part it will be ridiculously cheaper. Now other factors will influence - like missing pieces, unique situations, security concerns etc.