The average Mozilla Gecko engine developer can quit and get a job at a big tech company making a lot more than $200K a year, definitely if they live in the Bay Area, and probably even if they don't.

The total cost of an employee is quite a bit more than the pay, but yeah seems high?

Depends on the country and depends on the state. E.g. in California the basic overhead in terms of taxes payed by the employer is slightly below 10%. Then the personal income tax is 13%. You have to include other costs like matching 401k, medical premiums, other operational costs associated with each employee. $200k doesn't seem that outrageous from the employer POV since the employee is left with around $120k at the end of the day.

We're talking costs, not pay. You have to pay taxes and provide various benefits to meet the legal requirements of employing someone.