The most obvious reason is that it would sanitise all possibilities of free interpretation. Books often depend on an ambiguous plot: “was the baby hers?”, “did she commit suicide?”. We don’t want ambiguous Snicker ingredients, but often do want ambiguous plots, especially in genres such as mystery and detective fiction. It is not just an issue of spoilers; it is an issue of authorial intention becoming black and white. The solution of “don’t read the warnings if you don’t want to spoil it?” ignores that some may want to know if a certain topic is mentioned but would rather leave other topics unspoiled, which would be impossible under this system.
As triggers are just as diverse as a book’s content can be, the challenge is even greater. The possible common allergens in a Snickers bar are small, yet the possibilities of what could be triggering are almost infinite. It would be easy to accommodate for common sexual and violent content, yet sometimes, seemingly innocent content can be the worst for the wrong person, such as underwater or interrogation scenes. Ultimately, the system would always be inferior to searching “does this book contain X?” or using already existing websites that contain trigger warnings.
If the discussion ended here, I would still say it would be advantageous to implement. A faulty system is better than no system (unless you’re William Godwin). However, the impact on the reader has only been discussed, not on the supplier. Writers of brilliant novels may be rejected due to a publisher’s concern that some content would require a warning, which would reduce sales. This may lead to literature becoming ‘safer’, avoiding the treatment of triggering topics, which would leave the culture emptier and leave ideas unmentioned, leading to them stagnating in more dangerous areas such as online forums.
An issue of censorship also arises, both politically and parentally. A fundamentalist state would have a much easier job censoring non-permitted texts from their country with the handy categorisation suggested. Similarly, parents and schools would suddenly understand what Sarah J. Maas puts in her books, which would be a tragedy for pre-teens (and adults, no judgement) globally.
Finally, at a stretch, it could be argued that Newton’s Third Law would come in and lead to trigger-loving groups to thrive. Fetishists would love to be able to walk into Waterstones and see every book which contains content they desire. This may lead to publishers specifically publishing books containing weird content and writers supplying this demand. One Lolita is enough for a shelf, and I would rather publishers not attempt to turn it into a decalogy with Barnes & Noble leatherbound editions.
If there’s a simple solution to the problem, I’m not smart enough to think of it. Ultimately, like most things, I say we leave government-enforced warnings for Snickers and let the free(ish) market and the internet’s ingenuity solve the problem itself.