Let's say it takes 10 years for the unicorn to reach $100B and you've accumulated 0.25% equity per year for those 10 years (unlikely that you finish with 2.5% fully-diluted unless you were a founder, but let's go with it anyway). $250M cash after 10 years, but there is a 1/250 chance of this being the case, so the expected value of the $250M is (1/250 * $250M) = $1M = $100k/year over the ten years. Let's see if Facebook can beat that:
Even if you're E4 at Facebook for the whole 10 years (not even career level), you would average $264k/year, which is a guaranteed $2.64M over 10 years, excluding stock growth and promotions. Average E5 jumps to $385k/year. Source: https://www.levels.fyi/company/Facebook/salaries/Software-En.... So even the E4 option beats the expected value quite nicely, excluding promotions and stock growth (assuming the startup pays you less than $164k/year, which it likely will).
There are plenty of reasons to work at a startup, but given the choice between a top 5 tech company and a startup, you will 99.99999% of the time not make more money at the startup.
(Edited to fix math)