Lots of reasons why, including, your inability to recognize problems you are unaware of; the inexorable fact of reading what you think you wrote, not what is on the page; inability to see logical problems, missing assumptions, etc. Ad infinitum.
There are line-level hacks which can help with some of this, e.g. reading backwards to find typographic and spelling error...
...but there is no general solution.
Suggested fix: find a professional peer and become their formal editing buddy. Define terms and scope, this is not peer review–it is simply copy editing.