The equity value of a founder's shares is an order of magnitude higher than an early hire, or two orders higher than a late A or B round hire, and 3-4 orders higher than a pre IPO employee. No one gets rich as a regular IC hire at series B or C, even at a Google -- it probably tops out at 10-15mm, vs 15b for the founders. Even more so at companies less successful than Google.
On top of that, founders get supremely preferential tax treatment. Founders shares are generally bought near nil, and the entirety of gains are long term capital gains. If you do it right with QSBS and moving to WA before the sale, it can be taxed at zero. An employee often ends up paying hella AMT on almost all gain from ISOs since he didn't early exercise (or, early exercises at substantial out-of-pocket cost), and gets less freedom to sell in later financings, less return in a sub-optimal exit where the sale is covered with preference but "retention grants" are given to founders, etc.
I think the optimal strategy is to work for the most prestigious and successful companies purely as a learning and reputation building exercise, and then either be a founder or consultant to make money. It might be possible to enter a slightly earlier (series b) company as a director or better and still get a reasonable amount of equity (1%+ in a 100-200mm company), but otherwise either going for the tax advantaged, high variance outcome as a founder, or the high cash, flexible, also tax optimized outcome as a consultant, is the best way to actually get paid.