How much do you pay for equity (in the form of reduced salary), and what's the expected upside?
Venture Capitalists are in the business of making money off successful startups. They work day in and day out on finding startups that are likely to be successful and investing in ways to help them be successful. Most of the time, they're wrong. The only way that VCs are able to be profitable is by having a diverse portfolio of companies in the hope that one is successful enough to outweigh the losses of the others.
Picking out successful startups is (probably) not your expertise, and you only get about a dozen chances (on the high end?) to choose a new employer for the duration of your career. The odds are very very against you. Given those odds, we can value startup equity at approximately $0, perhaps a bit higher.
Unfortunately, most startups offer lower salaries in trade for equity. They also are offering you _options_ instead of actual shares. Unless you stick around long enough for a liquidity event that guarantees the option's value, you will have to exercise those options prior to knowing their true value. This costs you money _twice_ for an entity that has a near-$0 value.