Learning to speak and understand a language is a difficult task, and involves coordinating many physiological and mental activities, like pronouncing sounds that may be unfamiliar in different contexts, parsing constructions at speed, recognizing words from others' pronunciation (which is usually quite different from "standard"), and recalling their meanings.
Reading, on the other hand, is not language, but technology. There is no human adaptation to literacy as there is to spoken (i.e, real) language, and in fact some humans never manage to master literacy in their native language, while others find it very simple.
So it's not that it's easier, but rather that it's easier for you. People vary enormously in language abilities, and that's squared and cubed for writing.
Like all technologies, there are lots of arbitrary features in writing systems, and many systems to choose from. I think anyone finds Mandarin easier to learn to speak than to learn to read in characters, for instance. But it can be written in Pinyin as well as characters, though that's not standard, and if you can speak the language, you can learn to read Pinyin easily enough.
Similarly, /ɪf ɪŋɡlɪʃ wərɪtən fəniməkli, ɪtəd be iziər tərid/, after a little practice, at least. And with added spaces and punctuation. But all writing leaves out important aspects of real language, like pronunciation, rhythm, gesture, gaze direction, facial expression, and intonation.
Consequently virtually all written sentences are multiply ambiguous, like these Garden Path sentences, which exist only in written form; as spoken, there is no problem.