Well corporations pay printers to do their printing because they don't want to be in the business of printing. It's the same with infrastructure, a lot of corporations simply don't want to be in the data centre business.

That's how nearly every aspect of every business works; would you you start a bakery by learning construction and building it yourself?

Construction is a one time cost. It infrastructure is in constant use.

It's like accounting and finance. Yeah a lot of companies use tax firms, but they all have finance and accounting in-house.

I don't think that's the important distinction, lots of ongoing costs are outsourced.

Amazon may have their own shipping fleet, most retailers of smaller scale pay someone else to do it for profit.