The opposite of depending on another company to host a service is to host it yourself. The average company of any size has at least a dozen external SAAS dependencies. Do you want to have the expense of managing all of those services yourself?
Anecdote, but here is a specific situation. Company X has both internal cloud (based on OpenShift) and “public” cloud (Y: one of the big three cloud providers). Application team Z has applications on both, for historical reasons. One of these days application on the internal cloud fails, remediation requires attention from X’s infrastructure team (which is not the same and quite removed from application team), situation is dragging for hours initially and repercussions are cleaned up for several days afterwards. Business is losing money and is furious. Similar situations happens on a public cloud, application team gets a tech resource from Y calling back in two minutes and the whole situation is resolved in two hours.
There have been times that the control plane blew up in one zone of one cloud provider.
I had a least-complexity system which lived in that zone so it kept functioning as if nothing happened.
Other people panicked or had their automation freak out for them, they tried to restore their multi-AZ systems, overloading the control plane for the whole region.
It's a stressful experience if you feel you can't do anything about it, and sooner or later you will feel it with cloud computing: periods of 8-12 hours where your environment is FUBAR and you are best having some faith they will stitch it back together and walk away from it for a while.
If my colo goes down. My customers are going to complain and think I am incompetent. If my cloud hosted infrastructure goes down and everyone else is down your customers are a lot more understanding.
It depends on what you want but managing complex software may be expensive and has some serious economies of scale, which makes a pressure to outsource, even expensively. You can't just brush it off with 'that's easy', it's not.
You are asking for a solution before agreeing that there exists a problem.
This is the "I'll listen to your problem if the solution is convenient" mentality. It's inside-the-box thinking. It holds you back.