The short answer is domestic politics.

It certainly is not in our best foreign policy interests to anger most of the other countries in the Middle East. However, politicians are mostly concerned with reelection, and most Americans do not prioritize practical foreign policy concerns.

The most important reason why the U.S. unquestionably backs Israel is because of American evangelical Christians. Evangelical protestants tend to support Israel more than other groups because many of them want to make the end times as described in the Bible’s final book, the Book of Revelation, a reality. According to evangelicals, it is necessary for the Jewish people to return to Israel for the end times to begin.

Evangelicals may not be the majority, but they are very politically active and well organized. They may be only about 20-25% of the U.S. population, but they are 36% of all registered voters.

Most groups in the U.S. support Israel over Palestine, but the gap is clearly the widest with white evangelicals with 79% supporting Israel while only 5% side more with Palestine.

(The above images are both from Pew Research. They are a nonpartisan group that researches trends and does polling. Pew Research itself does not ever take any position on issues or policies.)

Money in the American political system also plays a significant role. The pro-Israel lobby is quite powerful. AIPAC, the American Israel Public Affairs Committee, is certainly one of the most powerful lobbying groups in the country. However, appealing to the evangelical base is still the main goal, even over appeasing the Israel lobby. This is especially true for the Republican Party where 45% of their base is evangelical.

To clarify, this support for Israel from evangelicals does not mean that they support Israelis or Jews in general. The fact that an evangelical preacher with a history of anti-Semitic comments was invited to the opening of the new American embassy in Jerusalem clearly shows this. It is important to recognize that current U.S. policy towards Israel does not prioritize the long-term stability of Israel or Israelis nor does it prioritize peace.

Sources

http://www.pewresearch.org/fact-tank/2016/03/14/exit-polls-and-the-evangelical-vote-a-closer-look/

http://www.pewresearch.org/fact-tank/2016/05/23/5-facts-about-how-americans-view-the-israeli-palestinian-conflict/

http://foreignpolicy.com/2013/11/01/how-not-to-think-about-the-israel-lobby/

https://www.usatoday.com/story/news/politics/2015/03/02/a-look-at-aipacs-influence-lobbying-in-washington/24257355/

https://www.nytimes.com/2018/05/14/world/middleeast/robert-jeffress-embassy-jerusalem-us.html