I don't think these comparisons are useful. Every time you look at companies like LinkedIn or Docusign, yeah - they have a lot of staff, but a significant proportion of this are functions like sales, customer support, and regulatory compliance across a bazillion different markets; along with all the internal tooling and processes you need to support that.

OpenAI is at a much earlier stage in their adventures and probably doesn't have that much baggage. Given their age and revenue streams, their headcount is quite substantial.