A late night monologue references things that happened that morning, which means it was written in the hours before taping. The schedule that makes this possible is unusually rigid.
Topical jokes have a short shelf life
A joke about a news item works only while the audience still has the item in mind, and attention moves within a day or two.
Writing further ahead risks a premise that has been superseded, or worse, one that has been rendered inappropriate by later developments.
The freshness that makes the form work therefore forces the deadline, since there is no way to bank material that depends on the morning's events.
Writers submit in volume
A staff generates many more jokes than can be used, each written to a premise drawn from the day's stories, and submits them by a fixed hour.
Selection happens quickly afterward, with the strongest jokes chosen and ordered into a sequence that flows rather than jumping between subjects.
The high ratio of written to used material is the mechanism that guarantees quality under time pressure, since choice substitutes for revision.
The rehearsal is a test, not a polish
A run-through in front of a small group reveals which jokes land, and the order can be rewritten immediately afterward on that evidence.
Lines that fail are cut rather than fixed, because there is no time to rebuild a joke and a replacement already exists in the pile.
This is why the monologue as broadcast is often shorter than what was rehearsed, with the weakest material removed within the hour.
Structure carries the weaker jokes
Monologues are arranged so that related jokes sit together, letting a strong opener establish a subject that subsequent lines can build on.
A joke that would fall flat cold can work in third position, because the audience already has the premise and does not need it explained.
Ordering is therefore a craft decision with real effect, and it is one of the last things settled before taping.
The delivery absorbs the failures
Hosts openly acknowledge a joke that does not work, and that acknowledgment is itself a reliable laugh, which converts a loss into a gain.
The convention exists because same-day writing guarantees some misses, and a format that could not survive them would be impossible to produce nightly.
Audiences have learned to enjoy the recovery, making the host's relationship to failure part of the show rather than an interruption of it.