Hosting has a minimum cost of entry.
It may make sense that for "Giant Corporation A (GCA)", hosting everything in the cloud is cheaper. BUT, for "Small Team A", within GCA, the costs cannot always be justified. I can buy a capable machine for $500 off the shelf, spend a day or two configuring it, make a system image, and then bam, I run that machine till it dies. Then I buy a second machine, it takes maybe an hour to restore the image and get the new machine back on the network. And I don't need to be paying engineers to do that effort either. Rinse and repeat as needed. That labor and up front cost is minimal compared to the monthly lease and up front setup cost of any hosted solution.
"You're employer is doing it wrong", you might say, and that's probably correct, but with so many divisions under GCA, it's impossible to provide a single hosted solution that can meet all the technical, security, and licensing constraints of every program. Just because Team A and Team B both work for GCA, doesn't mean Team A and Team B can host their data in the same environment.