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.
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.