Do you mean at
International School? It's probably because they are also expats in the main.
There's a high turnover of expat children at these schools too. As most teachers at international school are also expats, I don't think it's strange that many don't stay long.
Teachers at our school are usually either married to expats who were the ones who got the job here in the first place (a trailing spouse if you like), or they're singles who wanted to work in a few different countries for the experience.
It's not so different to the rest of us expats I suppose - we all have our reasons for becoming expats and most don't move abroad with the intention of staying in one place for years, nor do all enjoy their time as an expat. Many people do a year or two, decide they don't like it and move back home, some go elsewhere because they don't like the country they are in and some do a couple of years and then the spouse gets moved on and they have to go with them.
Our kids have never had a problem with it - their teachers changed each year anyway (even when they were in kindergarten) so it didn't matter if the teacher was new to the school, new to Singapore or had been here for years, it was always a new teacher every August.