Ask HN: Why do contractors exist if they charge the same as a FAANG FTE?

I recently worked out that a faang salary works out to the same as a fairly average skilled contracting rate (say $1k/day). What is an employer gaining by hiring contractors (who probably receive 25% of that themselves) instead of employing another full time person at 250k per year?

Are there other major costs to company of a full time employee that are significant relative to the salary?