Probably because there is no reason to think otherwise. Also, there are clues, like where Jesus in his story of the rich man and Lazarus describes it as a place of extreme heat.
Abraham explains to the Rich Man why he is there.
Do you recall the reason Jesus gave for the Rich man to be in hell?
Does that match your doctrine of why someone would be in hell?
If not, why would Jesus give that reason?
There may be good reasons to think the fire in hell is not literal.
Would a coal from a fire burn your lips?
Isaiah 6:5-7 NIV
“Woe to me!” I cried. “I am ruined!
For I am a man of unclean lips,
and I live among a people of unclean lips,
and my eyes have seen the King, the Lord Almighty.”
6 Then one of the seraphim flew to me with a live coal in his hand,
which he had taken with tongs from the altar.
7 With it he touched my mouth and said, “See, this has touched your lips;
your guilt is taken away and your sin atoned for.”