Companies often outsource to prove out a concept and then hire in-house once it's clear that skillset will yield steady revenue.

For example, clients often hire me and my team to create and manage digital marketing campaigns for a 3-6 month period, and once we prove out a consistent framework for generating revenue through specific marketing platforms, i.e. we demonstrate that if they spend $10k/month on advertising, they generate $25k in returns, the business can now justify an in-house specialist to manage this stuff.

Some companies still prefer to outsource at that point too, because buying an employee comes with additional costs: they need a desk and a computer, health insurance and benefits, etc. Contractors and agencies are a simple relationship where the cost is no more than face value and they can cut the cord with 30-day notice if something changes in the business, without having to worry about somebody putting food on the table for their family.