However, the private market liquidity is always controlled by the company, and that can create artificial boundaries on timing and volume, which can be trouble if an employee wants to leave on their own schedule.
Also remember that if your $5mm company becomes a $1b unicorn after 4 years, and you got %0.5 at the start, then through dilution your %0.5 stake can become a %0.05 stake. Which means you get $500k / 4 years = $125k/yr in stock. But you cannot sell that stock, so it would of been better to go work at apple. It's very rare that a startup will pay better for an employee better than the big cos.
Yes, as I noted above I do agree that volume constraints are an issue and are pretty annoying. Even in the examples where you get to hold options for 7 years, you wouldn't get the ability to sell at the "peak" (if you think there is one) unless the company was public.
There are many things you have to take into account when valuing stock options, and from a purely compensation basis I agree that Apple/Google/Facebook are going to be tough to beat.