These people deny the appearance of Dajjal and his Fitnah because they don’t find it in Qur’an by name. They also mock the traditions of the ProphetSAS that have come regarding this. See below link for their denial
Traditions of the ProphetSAS
The ProphetSAS has said, “Whomsoever denies the appearance of MahdiAS then he has disbelieved in that which was revealed to MuhammadSAS and whoever denies the appearance of IsaAS then he has disbelieved in what was revealed to MuhammadSAS and whoever denies the appearance of Dajjal then he has disbelieved in that which was revealed to MuhammadSAS”. (Fasl ul Khitab)
Narrated Abdullah bin Umar: Allah’s Apostle stood up amongst the people and then praised and glorified Allah as He deserved and then he mentioned Ad-Dajjal, saying, “I warn you of him, and there was no prophet but warned his followers of him; but I will tell you something about him which no prophet has told his followers: Ad-Dajjal is one-eyed whereas Allah is not.”. (Sahih Bukhari)
Abu Darda’ reported Allah’s Apostle (may peace be upon him) as saying: If anyone learns by heart the first ten verses of the Surah al-kahf, he will be protected from the dajjal. (Sahih Muslim)
