Same reason the human brain has a train of thought: there's a lot of stuff you "know" as episodic memories of words you've heard/read, but haven't yet internalized via practice into reflex reactions. It's a lot faster and cheaper to acquire a memory of some words, than it is to acquire an entire new skill, so you end up with far more memories-of-words than concrete skills.

Reasoning (i.e. bubbling those remembered words up into your mental context, and then using a special "acting as constrained by words" bridging reflex on them, a.k.a. "system-2 thinking") is how you leverage memories-of-words into useful behavior.

An especially useful version of this is planning, where you chain through "mere words" until you end up at words that declare to yourself what you're about to do; and then you rely on your bridging reflex to react in a way that's constrained by the plan you generated.