fast food is never healthy ! they were kept in cold freezer for months, deep fried, so much salt and sugar.
the reason why people kept going for it is becos of the salt/sugar taste which their body senses are addicted to ! plus the idea of eating out in a lifestyle concept, it is a very strong marketing tool to capture the young consumers market who are tech-savvy and conformed to trendy lifestyles.
well, macdonald's is actually one of the cheaper dining options in the US. you get a complete meal for US$5-6 compared to US$8+ for a burger in a diner.
they don't have hawker centres and food courts like us, so it's probably the wallet driving the stomach especially for the poor