Over time I suspect that will change as more and more people learn the questions they should ask when evaluating an offer, i.e., as they transition from naive price takers to more sophisticated price setters. This link was circulated on HN a few weeks(?) ago, but it's worth reposting: https://github.com/jlevy/og-equity-compensation.
And here are some companies that offer >90 days: https://github.com/holman/extended-exercise-windows (e.g., Coinbase and Pinterest with 7 years, Quora and Asana with 10 years, etc.). Interesting to see two companies in the list founded by early Facebook employees -- well, cofounder and CTO -- do longer periods. Wonder if people got burned there by the standard period?