Why are 'open-source' content management systems better?

The content management system we use is designed and built by a global team of developers - literally hundreds of people contributing to both the core system and the hundreds of modules that add all kinds of extra functionality.

By contrast, an agency that has built their own content management system will typically have 2-3 developers working on it at any time. There is simply no comparison in terms quality, stability and functionality between the two options.

Open-source is the future - agency-brewed is the past.