You're right, and I hit on that at the end. Problem is twofold:
(1) You get the most by being a very early employee -- like #1 - #5. It's a hockey stick distribution here. The later you get in, the exponentially lower your stake will be.
(2) The most successful startups are outliers.
Getting in that early means you do not have a large team or a long track record to look at when making your decision. The big bucks will come from the coffee shop hobo or ramen-snarfing grad student outfit with 1-3 full time people (all founders), barely any capital, and no track record. People will laugh at you and they're probably right.
Probably.
But if they're wrong, you will laugh more later.
If it has success written all over it, it will look "hot" and "cool." That means there will be a ton of competition for those early positions. You will be edged out by someone with a Magna Cum Smartypants degree from an Ivy League school. It's also probably going to be so capitalized (since it looks "hot") that they'll just skimp on the equity and pay more, which invalidates the thesis here and makes it just a regular job from a purely financial risk/reward perspective. (It still might be worth taking for the experience and fun factor.)
That's why if this is the strategy you want to take you are more or less doing what seed stage angels do. You are looking for feral unicorns that poop unpolished diamonds. If everyone else thinks it's a good investment too, the market will behave "efficiently" and you will not get rich. The market doesn't "want" you or anyone else to be rich. Rich people are outliers.
Google and Facebook are great examples by the way. They both looked stupid: "another search engine?" and "so it's like MySpace and Friendster but with no capital and some Harvard dropout nozzle running it?"
Edit: the reason your equity/stake drops off exponentially the later you get in is precisely because of the stupid unicorn factor. Really early stage companies have to offer you a fatter option package because you're an idiot to work for them and they know it. There is no other way they can compete for talent. If they're successful, those options turn out to be very expensive -- they come out of the founder's equity pool.