Because they are in greed of power. As Galadriel said in the prologue, "who above all else, desire power". 😄
Sauron deceived them. Promised them long spans of life, maybe even immortality and ongoing wealth and eternal power. Then they accepted the Rings and they became great indeed during their time just like Sauron promised, but still, he claimed his price eventually. 😄 They slowly fell into the Shadow and became Sauron's deadliest and most loyal servants. The rest is history. 😄