Quoting another writer's work is not automatically permitted, and clearing those quotations is a task that falls to the author near the end of a book's production. It affects what appears on the page more than readers realise.
Fair use is a judgement, not a rule
Copyright law allows limited quotation for purposes such as criticism, review and scholarship, but it does so through tests weighed case by case.
How much was taken, how central it was to the original, and whether the use competes with the original all matter, and none of them has a fixed threshold.
Because the outcome is uncertain, publishers apply internal guidelines that are more cautious than the law strictly requires.
Those guidelines vary between houses and between countries, so the same quotation may be cleared in one edition and cut from another.
Song lyrics and poetry are the difficult cases
A short work suffers proportionally more from quotation than a long one. Four lines of a novel is a fragment; four lines of a poem may be a substantial part of it.
Lyrics are the most restrictive of all, since rights are usually held by publishing companies that license commercially and charge accordingly.
Authors frequently discover the cost late and remove the quotation instead, which is why so many novels describe a song rather than quoting it.
Clearance takes longer than it looks
Permissions require identifying the current rights holder, which may have changed through sales, mergers or inheritance since publication.
Requests then specify print run, territories, formats and languages, because rights are licensed separately for each of those.
Replies can take months, and some rights holders never respond at all, leaving the author to choose between waiting and cutting.
The bill usually reaches the author
Most contracts make permissions the author's responsibility, both to obtain and to pay for, sometimes with a capped contribution from the publisher.
Those fees are paid before any royalties arrive, and for a heavily quoting book they can consume a meaningful share of the advance.
The work is also unpaid time, arriving at the stage when the author is already handling proofs and every other final task.
What this does to books
The result is that quotation is rationed by cost and administrative effort rather than by what would serve the argument best.
Writers paraphrase where they would rather quote, and works whose rights are cheaply cleared appear far more often than works of equal relevance whose rights are not.