Word based passwords are (typically) not more secure, but they're easier to remember than random text, which makes them more secure than the lesser alternative, but not more than the better alternative: just as long, but fully random text stored and generated by a password manager. You're right that substituting text with numbers or symbols is bad, those are easily cracked. But fully random text with symbols and numbers is the best.

Why? Badly remembered passwords are often reused and written down, sometimes even on the computer itself, in emails, chat software, text files. And any password created by a person inherently will fall victim to shortcuts, as humans are often just not as creative as you might hope, there's patterns. Common words might be used, too little words might be used. With a dictionary attack that checks common words, the entropy of such passwords can become drastically smaller to the point they can be cracked very quickly, and you have to be aware of that constantly when using words. Using uncommon words or more difficult words can prevent that, but that's typically not what people do, when "password123" is the best they come up with otherwise.

A notebook at home would suffice, but it's not great for the same reason as word based passwords. A password manager can create passwords that are guaranteed to be entropically complex passwords that can't be cracked basically ever. There's no guessing, no shortcuts to take, no human laziness to slip in and curse the password to easy cracking. And it does so uniquely for every login you have. That's essentially unbeatable.