Employer tax contributions

Office space

Hardware and software, SaaS licensing, cloud costs, etc.

Hiring costs (recruitment, recruiters, time lost in selection and hiring)

Secondary cost to rest of the business to change processes, retrain, integrate, help the dev team understand requirements, effectively build and iterate, etc.

Quite possibly a bunch of compliance, security, audit, pen testing, and other regulatory costs depending on the demands their clients have, etc.

Running a team != hiring a bunch of freelancers as a one-off.