It takes 1 person a few percent of their time to manage something like that for internal use only. And you still get the gains of keeping secrets internal and cost savings on licensing costs which might actually pay for a good chunk of the effort in the case of running an existing service.
Compare with a small development team for a significant percentage of their time to build a halfway decent chat tool.
Compare with a small development team for a significant percentage of their time to build a halfway decent chat tool.
Miles of difference.