My friend who is Korean and can't speak English well got a TEFL certificate recently. She went to the TEFL school for two months, paying about ~$5,000 for it, and at the end of it, she got a certificate. She is now qualified to teach English in Korea. The TEFL school is run by an American university. 

Anyone who hears her speak knows she can't speak English well. Strange pronunciation, strong Korean accent, wrong grammar. How can she get a teaching certificate when she doesn't even know how to speak English? She only did that course because a lot of her friends (all rich bored housewives) did that course and got a certificate. They all cannot speak English well let alone teach it. Because people like her see that it's easy to get the prestigious certificate, they pay money to that institute and do the course.

The institute gets a lot of customers and the housewives are happy because they have a piece of paper telling them that their English is good (but it really isn't). It's like a scam to make money off wealthy housewives with a lot of time on their hands for whom having the ability to speak English well is like a status symbol, like owning a Gucci bag. English ability is the new status symbol in Korean society. Even if they can't speak English well, these people still like to show off and pretend they can. Korea is fertile breeding ground for scammers in the English teaching industry. 

You can't trust certificates (with exceptions) anymore. Because some TEFL/TESOL institutes want to make money from having a lot of customers, they pass people like her and give them a certificate. 

The only qualification I would trust is the TOEFL score. You can't cheat your way to getting a high TOEFL score. The TOEFL examination is marked anonymously and according to strict objective methods. Anyone who cannot get a score above 115 (out of 120) should not teach English. 

Sometimes non-native Speakers pose themselves as native speakers to get more studetns :)

It is a fraud!

I agree that this is not fair, and that ideally English teachers should be judged on their merit. Indeed, it may be the case that a qualified non-native English speaker may be a better teacher than a non-qualified native speaker. However, in general settings (leaving out qualifications), most people consider that native speakers will have the strongest grasp of a language, its nuance, its colloquialisms, etc. In many cases this is true, but of course there will always be exceptions to this rule.

What this has done is create a "premium" with being able to advertise that you have actual native speakers teaching, despite this not necessarily meaning that they are better teachers. Because the market now places a premium on the native tag, and because there are so many native English speakers, it will unfortunately be more difficult for non-native English speakers to find work as English teachers. However, opportunities will be greater in countries and regions where there are less native speakers available wanting to teach.

From English learners' perspective, one of the biggest challenges is how to express their ideas or feelings in English naturally but the problem is they don't know what is natural to native speakers. That's why native English teachers are more acceptable to them.

For me, as I'm living in Australia, I find it's really strange that many schools here let non-native teachers teach English to international students. Maybe they think that that would promote multiculturalism but I'm not sure if it's a benefit to the overseas or it's just a rip-off.

Nativity is probably the easiest way to assess a speaker's language ability - not necessarily their teaching skills, though. I found my best German teacher was a non-native speaker because she understood the mechanics of learning grammar and didn't just brush things off when learners struggled with unfamiliar things. The native teacher was sometimes unable to explain things because she relied on "feel" or "intuition".

Often a non-native teacher knows the challenges faced by the learner and has more sympathy. So, in that sense, I think the reluctance to work with non-native teachers is because learners are not sure (being new to the language) if the teacher has great pronunciation or not or whether the teacher really has fluency and will avoid teaching mistakes.

I find I keep looking at the native German speakers as potential teachers here - guilty of the same problem you bring up!