There are people who are close to retirement, so leaving would jeopardize their pensions. There are people who have families, and may fall into unhirable categories (anyone over the age of 30, especially if they are in their 50s).
This is not an economy that is conducive to living wage salaries. Many people are trapped because they simply can't afford to just jump and hope for the best.
You also have to remember that not everyone hangs out on this site. Maybe folks are coming here for the first time.
For better or worse, most employees of Sears are going to end up in a really bad spot as the company slides awkwardly into Chapter 7 because the economy is just not robust enough for folks to easily jump ship.
If folks really want a mass exodus, the way to do that is to facilitate connections between employees and hiring agents in other companies. If you're a former Sears employee and you know there are opportunities where you work, spread the word. If you can, put in good words for people. Most employment opportunities arise from connections, not cold-calls.
If you're not going to help people find new work, then you really can't complain about people not leaving. It's not their job to risk their own bankruptcy and homelessness just to satisfy someone's outrage over corporate malfeasance.