1) The weather. In places like Japan or Taiwan where there are four seasons, you need suitable clothes for each season, so you've got more mix-and-match options. Also, it's so hot in Singapore, I doubt many would bother to layer/dress up, and die in the sweltering heat. Even if we would like to, we'd risk being labelled "vain" by friends. "So hot still dress until like that! Just want to look nice only!"
In Japan's/Taiwan's/Australia's colder autumn/winter/spring weather, people would already need to layer their clothes to keep warm, so with good matching sense, people would be able to look really good and it would suit the weather. In Singapore, it's summer all year round, and people would laugh at you. For example, I like boots, but it's far too hot in Singapore to wear them (and not be laughed at by people dressed in more comfortable footwear).
2) Other stuff to think about. Sleep. Who cares about looking good in Singapore? If I'd only gotten 3 hours of sleep the night before due to homework, I wouldn't bother spending time looking good to go out. I'd rather use the time to sleep or something, and then grab a tee and jeans/shorts when I need to go out. Nobody would care about what I look like.
(The typical Singaporean mindset, no?)
Also, being the busy Singaporeans we are, we'd have a lot of other things to do, like homework, or housework, or work, or...catching up on sleep. Spending effort to look good isn't a priority. In fact, to some, it's a waste of time. "Look good for other people to see for what! Might as well spend the time doing something productive."
Are flip-flops acceptable? Acceptable in beaches/barbeque gatherings, of course.
Slightly unacceptable in shopping centres/areas, very uancceptable in wedding dinners (HAHA) and official meetings/gatherings.
The way you dress when meeting someone, I feel, also reflects how much you respect that person. If you go for dinner with an old friend in a reputable restaurant in a polo tee and FBTs, it implies you don't really care about the dinner with your friend, and that you don't really treasure the time spent with him/her.
What do I feel about people wearing their PJs out?
Wearing your pyjamas out is, in my opinion, unacceptable. No explanation needed, and I can't explain it. It's just.. plain disrespect. Like "I couldn't bother looking at least acceptable to go out and meet you, so I just wore my jammies out." But then again, it could also say "I know you don't judge me by what I wear, and you know me better than that, and I don't have to bother looking good to you, because you know me so well that looks don't matter." And that's good, isn't it? It's also a true test of friendship when you don't mind being seen in the streets with your friend who's in pjs. HAHA.
Then again, it depends on what you wear to sleep. I wear a class/CCA/camp tee and FBTs. But when I go out I make it a point to change, even if I'm going to wear another class/CCA/camp tee and another pair of FBTs. It's become a habit I guess.
I doubt a lot of people wear actual PJs to sleep in hot and humid Singapore. I'd drown in sweat. Unless you switch the air-conditioning on all the time, of course, but what happened to being environmentally friendly? Save the earth!