Well they can't, their passports are withheld until the contract is completed usually 2 years, recently they forced employers to let the maids keep their own passports, but alot still dont follow this rule. There is also a penalty if the maid leaves before their contract is up. Maids are like prisoners in the house, they can't even go outside at night after work, some don't let them use their hp. How do I know? I live in Singapore before and my wife is this kinda control freak. I used to secretly let my maid use the wifi and phone and I was adamant to let them rest and go out. There is alot of unknown abuse mentally, pressurising them not to go out and insisting them to rest at home.
In China, the maids or aunties as we don't call them maids are totally diffeent, i dare say China respects labor more than these countries. They are paid 2 to 3 times more, stay on their own and have their own private lives even if they stay with the employers. They can leave wheneve they want. They act as if i owe them money if i don't give tjem red packets during Chinesse New Year. Lol