I think another factor is the salary sites such as salary.com, payscale, glassdoor, etc. These sites often don't include RSU in comparisons among companies. If it was included, many could see just how underpaid they are and there would be more pressure on companies.
My guess would be at the vast majority of tech companies that are not called Google or Facebook, RSU compensation is a tiny add-on to your base comp and wouldn't move the needle much.
