You need to deal with overhead. Nobody does their own HVAC in house because you rarely need them, and would have to pay to train people on that despite them not using it.

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.

Lots of places have their own HVAC techs in house, if they have enough HVAC work to justify it. Even if it's not their core line of business. They will do whatever costs less, +/- some amount of subjective "hassle factor."

Especially when it's "line critical" to their business, or if the person can do other things as well.

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).

I have dealt with a large number of retail colo providers, wholesale data center providers and corporate owned data centers across the US over the last 20 years and all of them used contractors for HVAC and electrical. I'm not saying dedicated staff never happens but it is definitely not the norm.

Doesn't this logic apply to pretty much everything? Why hire external anything then? Why not do your own deliveries, hire your own trucks to transport goods etc?

There is a cost to taking on things that aren't part of your core business too.

All I can come up with is "Because economies of scale". I work for a transportation company, but we employ plumbers, carpenters, electricians, elevator repairmen, and many more that I'm not aware of, because we have enough locations / work to justify them. The pizza place has enough work to justify hiring a fleet of drivers, Amazon ships enough crap to justify having their own trucks (when they can't sucker another company into taking the unprofitable routes).

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.