The system exists because of the politicians, and the people who elect them. Many teachers do NOT approve of the system, which is why teachers have such high turnover.
What is your evidence of high teacher turnover? The turnover rates are heavily influenced by the school district and service years. Inner city schools are battle zones so the turnover may be much higher across career stages. In surburban school districts, the turnover is low especially as tenure is gained and service years increase.
The nature of the work and the compensation structure ensure low turnover. For young teachers, preparation is difficult. After teaching the same classes for a number of years, preparation is much easier. Pensions provide a large amount of deferred compensation that is lost if a teach retires before eligibility for full pensions. The pension structure also ensures that many teachers retire young when reaching the pension eligibility age.
Teachers are unhappy with the system, when the system in question is the school system. But, like other government employees, teachers are overwhelmingly leftist in their political leanings. They support the principle of government taking responsibility for people's lives -- they just disapprove of the implementation.