Reading a recipe doesn't mean you can bake a cake, however baking a cake without a recipe might taste like.......

Reading books and learning pure theory won't make you a competent teacher I agree.... however the courses DO provide some insight into the methodology, provides examples of the do's and do not's of teaching, offer drill and games/activity ideas plus assist with lesson planning. The TEFL/TESOL/CELTA courses let you hit the ground running when you step into the classroom for the first time plus offer job placement services.

I've been an ESL teacher trainer for some years now, and am well aware that the teachers who arrive with qualifications pick up the basics faster, are familiar with the general idea of what to do and are able to settle in to their job in a far shorter time... personality aside, because some people are just naturally better. Those people who were 'born teachers' are the exception to the rule.

Example: Imagine if you were hiring an inexperienced chef via email. Would you take a qualified person with documentation proving they have attended cooking classes... or just some guy that claims he can cook but has no way of proving it? Easy choice for recruiters. The best schools pay top dollar to recruiters for the best/first pick of applicants (qualified, native speaker, experienced), the rest of the applicants can be shipped off to the kind of places you read about in blacklist forums if they're unlucky.

I'm not saying without qualifications you can't teach well, experience far outweighs and certificate... though I want to point out if you want a higher starting salary, more career opportunities and a chance to work at top quality reputable schools you need to look at becoming an internationally qualified ESL teacher first. Unless you're just in it for a year.