Reading a language you can already understand is less complex to learn than understanding a language in the first place
Less complex in some abstract sense doesn't neccessarily mean anything about how actual brains do it. Babies and young children are pretty much hard-wired to seek and acquire the structures of natural language, and that doesn't translate to other systems with different characteristics.
Here's an interesting case in point: Little kids will naturally acquire a signed language that they are exposed to (by fluent signers, I'm not talking about parents who do "baby sign"), because it shares the structural characteristics of spoken language; but they do not acquire "signed English" as a natural language, because it is awkward and artificial and the pacing is all wrong. They can learn to do it, but it is more of an explicit learning task, like learning to do math, than it is like natural language acquisition.
Learning to read is in this category too. For most kids. Don't forget we have an extremely biased sample here on this board!
