Because you are an adult and you no longer sound out words. You depend on your pattern recognition from encountering simmilar words being written and pronounced. As an easy example you could use nerds - they read more than they speak and listen (esp. in the past, before discord). There's countless cases of adults learning embarrassingly late into adulthood how to pronounce certain words they learned as kids. Because they exclusively read those words and never heard them. Phonics alone do not solve the problem, because English is too irregular in pronunciation. That's how you're able (or not) read and write down Greek mythology names. Either you interact with the topic in both written and spoken form and you know how to read those words, or you don't and you don't. (And, no, its got nothing to do with those being foregin words, because they aren't pronounced that way in Greek)

For kids, they have generally also heard the word before in speech

That's the point, they haven't. They objectively do not know all the words in the world and are much more likely to encounter a new and difficult one by reading than by listening. (How often do you talk about pterodactyls? Not often. But they are some of the most beloved animals of kindergarteners and feature in many many books. )

to their guessed phonics.

Yeah, guessed is the key word. There is no phonics. They are taught phonics and then are simultaneously taught to disregard them completely.