Downpayment can be paid using cpf. Assuming your monthly CPF contribution is 1k (which isn't a lot considering there's employee and employer portion), that's 12k a year without including in bonus. If based on CPF account allocation (OA, SA, MA), it's 23% of salaries that can be used for housing downpayment.

Working 4 years there's already almost 50k CPF for downpayment for just one person, and usually housing is funded by two persons. I would say that's achievable in the late 20s

The comparisons are not apples to apples. You have Singapore where you took only the resale segment of public housing, you have hong kong data set which includes both public and private housing, and the restrictions on hk public housing resale are more restricted than that of singapore.

And this is just the two countries I'm more familiar with. I don't have much confidence if you are using the right comparison for US if the two out of two I'm familiar with are wrong