It's hard work, and the pay tends to be far below the amount of work expected. Pre-school is exhausting, and grade school and high school often demand as many hours of support work, meetings, after-hours activities, and lesson preparation as hours of actual classroom teaching Many of those teachers also hung on through several deep recessions, and have reached retirement age or worked well past retirement age. And many "district" educational boards are encouraging senior teachers to retire early, so that younger, teachers with no seniority and lower hourly wages can fill those roles. Older teachers often disagree with the latest fads, and have the experience and knowledge to resist fads: middle management often finds those older teachers to be a dangerous "note of discord", and work politically to eliminate them quietly.
Please note that most of those issues occur in senior engineering roles. In IT, the sudden egress of senior tends to be much faster, and more concentrated to single companies.