That's patently untrue.
Written languages are either phonetic or character based.
Chinese is an example of a character based language. Each symbol represents a word. A reader coming across an unfamiliar symbol needs to guess what it could represent based on the context, ask someone or look it up.
European languages are phonetic. Each symbol or group of symbols represents a sound. A reader coming across a unfamiliar word can use their phonic knowledge to work out what sounds the letters are most likely to represent and then blend them.
Much less strain on the memory than having to memorise a symbol for each word, although fluency leads to words which are seen repeatedly being recognised rather than having to be decoded.
The misconception that the written English language isn't phonetic is because we have a complex code.
Some languages such as Spanish have a simple phonetic code where a written letter nearly always represents the same sound and where a sound is nearly always represented by the same letter.
Unsurprisingly, the rates of dyslexia among native Spanish speakers is lower than among native English speakers.
English has a complex phonic code because our history of invasion and immigration has meant that the language has been influenced by very different languages.
Anglo Saxon had Germanic roots but the Normans brought French with them. Other languages have also had an influence.
"ch" represents one sound in cherry and cheese but a different sound in words borrowed from the French such as chef or chauffeur and yet another in words derived from Greek such as chasm or school.
Flemish printers introduce the h into words like ghost and ghoul so that they looked more "correct" to people who were used to Dutch spelling.
There are countless other examples.
So English absolutely is a phonetic written language but a very complex one.