In the same way their written language couldn't express vowels, the english written language cannot adequately express the full range of tones and pauses we have when speaking.
People have attempted to convey some portions of spoken tone with thing like the "<sarcasm>" tag, and commas/ellipses/etc all convey some level of pauses, but they are not substitutes for the full nuance of spoken english.
Asking "Why did their written language not convey the full range of spoken sounds by default" is like asking "Why doesn't written English contain volume cues to indicate when the speaking tone should be louder".
All written languages that are created to represent a spoken language are inherently lossy.
they are not substitutes for the full nuance of spoken english.
A good illustration of this can be found in live speeches or interviews. A transcript often seems confusing or disjointed. When you view the actual speech itself it's cohesive and easily understood.
An even more direct comparison might be something like "I read that book," which has two different meanings that are pronounced differently but spelled the same.
People have attempted to convey some portions of spoken tone with thing like the "<sarcasm>" tag, and commas/ellipses/etc all convey some level of pauses, but they are not substitutes for the full nuance of spoken english.
Asking "Why did their written language not convey the full range of spoken sounds by default" is like asking "Why doesn't written English contain volume cues to indicate when the speaking tone should be louder".
All written languages that are created to represent a spoken language are inherently lossy.