Their employees get a share of the profits (like many regular businesses). They are also quite firm in their goal to stay private, so why would equity be relevant to an employee any way?
The share that goes into your yearly salary, bonuses, raises, and benefits (like the 401k match you disregard).
They owe you nothing else, and each employee is free to ask for more or leave for greener pastures if they are unhappy.
Also shares keep on producing income after you have left a company.
If I was making $120k per year working at MailChimp the last thing I would want is 30% or more of my net worth tied up in private shares. What employee wants that much exposure? It’s a private tech company with no plans for liquidity potentially ever.
Nobody is forcing people to work there.