Everything GM said.
Also, to put something else into context, I remember being open-mouthed and appalled when I realised that there is a fixed (minimum) rate of pay for certain nationalities, and so for example, as far as maids go, the pay scale goes (roughly)
Filipina
Indian
Ethiopian
Sri Lankan
Indonesian
or it might be a different order - can't quite remember! But it's all about supply and demand. The minimum rates tend to be set by their own governments as conditions for allowing them to come and work here en-masse, not by the UAE or Qatari government. So when manpower agencies bring them in for work they have a package pre-arranged at home - just like my husband did, and it is based on their experience and skill sets, and what their own expectations are, and what makes it worthwhile for them to come here compared to staying at home - just like it was with us.
The Filipinas tend to earn more because they speak much better English, are more likely to be literate or to drive, more likely to be happy to live with Westerners' dogs, more likely to be Christian and therefore find it easier to adapt culturally into a house of northern Europeans or Americans etc, they are more clued up about what standards we would expect with child-rearing, and they are more likely to have relevant work experience, and less likely to abscond. They also have very wide networks of friends and family here which keeps them sane and happy, so it is safer investing the (considerable) sum to sponsor them. Some of the other ladies from the other countries can struggle with all of that, so you get what you pay for, like anything in life.
And as the Westerners (sorry to keep using that phrase but it's a useful catch-all for Brits, North Americans, Northern Europeans and Antipodeans!) tend to want those qualities, and are more generous when paying for them than the Arab and African and Asian employers who just want slaves that's what the ladies from the Philippines get.
They are much less likely to want to work in an Arab household where the pay and conditions will be worse - but they rarely need to, and the Gulf Arab families prefer muslims (usually Indonesians and Malaysians) anyway.
And as far as Indian/Pakistani/Filipino expats bringing in their spouses and children - yes of course they can. But the cost of living here is cripplingly expensive for them (and pretty damned expensive for us too actually - rent and grocery shopping are easily on a par with London) so whilst it does happen, they would often rather keep their families at home, visit each year, and send the money home - where it goes a great deal further. Because after all, money is why they are here.