Because they don't work and are only prescribed by doctors in SA because that's what they're told to do. Doctors in zim(many many malaria areas) will specifically advise against it.
The pills mask test results so if you get malaria you test negative and therefore aren't treated, had a friend this happened to and he ended up with cerebral malaria because doctors refused to treat the malaria because he took the pills and tests were negative.
Use sprays, nets etc as they are just as effective as the pills and wont mess up tests results. If you do catch malaria the treatment is very effective and you'll be up and about relatively quickly.
I always hear this masking story but I have not met a doctor that has stated anything else than take the meds and that includes doctors that go to malaria areas often. Personally I wont chance it and telling someone on a forum especially someone from overseas where they have no notion of just how dangerous malaria is is reckless IMO.
If you have a study where they state its better not to take the medication I would love to see it but everything I see says the prophylactics are effective.