The downside for saving the costs is that you're losing control with every step taken away: as soon as you go into a datacenter of any kind, you simply cannot call up a HVAC company and offer them 100k in hard cash if they're showing up in the next 60 minutes and fix the issue. With a colo DC you can usually go and show up there to see if the HVAC, UPS and other systems are appropriate to your needs, but with one of the big cloud providers you have to trust their word that they are doing stuff correctly.
Now I'm deeply confused. Any company hired either has a profit margin (plus enough to fund an "Oh shit" fund in case times turn bad) or will not stick around longer than a few years. At which case why not just hire people directly and cut out the other company's profit margin? Assuming you hire similar people at the same rate, using your own existing and already-paid-for HR, how is that not cheaper?
In some cases you can even get a discount. Utilities are. Big customer of tree trimming, the companies doing that work can give a great deal because the utility doesn't care that they take a week off after a storm for high profit margin consumer trimming.
Larger hotels often have dedicated staff for things like HVAC, etc, because the importance of getting things fixed quick if possible is worth the cost of having someone onsite/available.
And you see similar things with colleges, etc; they often have a maintenance deportment that can be pretty large (though no doubt they've spun it off and brought it back in-house for the same "change is progress" reasons).
There is a cost to taking on things that aren't part of your core business too.
Similarly, Google doesn't ship enough stuff worldwide to justify drivers, insurance, trucks, jets, etc. - Fedex has the size and scale to make every package a couple cents cheaper, so it's just not worth it for Google.
The only other argument I can think of is the challenge of keeping every plate spinning, in good times and in bad. This is where your point of having a cost to take on something outside your core business comes in, but we seem to be in an era of mega-corporations - I'd expect lots of companies to snake tendrils into whatever will save them a fraction of a cent every time they have to do something.
If the AC breaks at 3am it neds to be fixed. It doesn't matter if you have your own HVAC people on sight 24x7, your own people on call to service it, a local HVAC service to come in, or you outsource the entire operations and so you have no idea how that is handled. In the end the important part of this story is that whatever you are doing with the AC continues to work. Different operations demand different levels of service (I doubt that anyone keeps HVAC techs on staff 24x7, but if the AC is that critical it is mandatory). The only case where the CEO is up at 3am is if the CEO is the owner of the local HVAC service company, not the CEO of the building with the problem.
Once you realize that to management the cost is outsourced no matter what the only question is do it with your own people and HR, or outsource it. There are pros and cons to both approaches, but for most companies it isn't their business and so the only reason to do it in house is they can't trust any company they hire.