As has already been pointed out in a comment and in one particular answer, there is a danger of mixing up things said by Jesus in chapter 5 of Matthew's gospel, which will lead to misunderstanding. To expand on that, here is the comment in a study Bible re. Mat. 5:22.
"Hell - The Greek word is ge(h)enna, which derives its name from a
deep ravine south of Jerusalem, the 'Valley of the (Sons of) Hinnom'
(Hebrew ge hinnom). During the reigns of wicked Ahaz and Mannasey,
human sacrifices to the Ammonite god Molech were offered there. Josiah
desecrated the valley because of the pagan worship there (2 Ki.23:10;
see Jer.7:31-32; 19:6). It became a sort of perpetually burning city
dump and later a figure for the place of final punishment." NIV Study
Bible, p.1420, 1987 edition
When Jesus then gave the account of the beggar, Lazarus, dying and finding himself in bliss (it doesn't say 'heaven', note), and the rich man dying and finding himself in torments, Jesus again used the word 'Hades'. The footnote on Luke 16:23 makes that point, adding:
"Hades is the place to which the wicked dead go to await the final
judgment. That torment begins in Hades is evident from the plight of
the rich man." (Ibid. p. 1541)
There is something intriguing said about hell and an eternally burning lake of sulphuric fire in Revelation chapter 20. Before the resurrected dead are judged at the great white throne, when Earth and sky have fled from the presence of the one seated on it, we are told that:
"...the devil that deceived them [the nations] was cast into the lake
of fire and brimstone, where the beast and the false prophet are, and
shall be tormented day and night for ever and ever." Revelation 20:10.
A.V.
Then comes the judgement of all humanity, and those whose names are not found in the Book of Life go to join the devil and his agents in that lake of eternal fire. The sequence of events is written like this:
"And the sea gave up the dead which were in it; and death and hell
delivered up the dead which were in them: and they were judged every
man according to their works. And death and hell were cast into the
lake of fire. This is the second death." Revelation 20:13-14 A.V.
Death and hell being cast into this lake of sulphuric fire, indicates that death and hell cannot BE that lake of fire. Yet there remains no doubt that the eternal state of those whose names are not found written in the Lamb's Book of Life endure eternal torment. It's not a matter of location; it's about condition, yet the wording here is very particular to show a downward spiral from Hades (hell) to an eternal state from which there can be no release via death, for this part of the Revelation applies to when time has ended, and eternity obtains.
As for Matthew 5:26 - that is a different matter, an illustration showing the need to repent before it is too late for the wrong-doer to avoid judgment. We all have until our deaths to turn repentantly to the Judge of all the Earth, to receive his free pardon, in Christ. But when we die (or, when Christ returns in glory - whichever happens first) the Day of Salvation is, for us, ended, and the Day of Judgment awaits at the Last Day.
Conclusion: The basic "reason why hell is considered to be a permanent situation" is because the Bible clearly teaches permanent torment for those judged as not being in the Lamb's Book of Life ("the smoke of their torment ascendeth up for ever and ever: and they have no rest, day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name" - Rev. 14:11 etc.) But because of inattention to the full scope of Greek words used to deal with the situations of the dead, before and after their resurrection, the English word 'hell' has been used all over the place, when the nuances of the various Greek words ought to have been taken into account. What should be clear, however, is that there is no soul annihilation where no possible awareness of anything exists; the everlasting joy of the blessed dead is contrasted with the everlasting torment of the cursed dead - in the Bible - call that condition what you will.
