It's simply impractical to unambiguously denote the music in such detail. Because even with the indication that one note should be louder and another quieter, it immediately begs the question of "how much?".
It's worth noting that many of the world's music traditions are passed along orally/aurally, which allows for a much more exacting reproduction of "how a piece should be performed" than leaving it to written instructions alone.
This is an insightful observation about the limitations of written music: there is a huge amount of information left out and therefore open to interpretation. Not only can loudness/softness change from note-to-note, but so can tempo or articulation.
Even when indications are given, they are ambiguous. A passage is marked forte, but exactly how loud is that? Should that forte be a specific decibel level? Should the note decay after the initial attack? At what pace should the decay happen? And what if the room acoustics change? And is forte in one part of a piece the same as forte in another part? How much louder than forte is fortissimo?
Sheet music, ultimately, is only a guide to what to play, which is why many musicians research pieces to get indications of how the composer performed them, or how people with direct contact with the composer (e.g., students) played it.
This "problem" of leaving so much open to interpretation has been a preoccupation for many composers. Early music was often understood as just a sketch, with musicians expected to improvise around it. By the time of Bach, composers were beginning to notate exactly what they wanted to be played.
Beethoven is credited with being the first composer to greatly emphasize the placement of specific expressive markings, with the idea that they were to be performed "exactly".
In the 20th century, composers of the "total serialism" school made an attempt at absolute control over every aspect of the music. On of the approaches most prominent composers, Pierre Boulez, would later acknowledge that the music didn't work, though going through the experiments was necessary — if only to discover that it didn't work.