I would guess it's a combination of (at least) two things:
- international credit boundaries. You could "buy a house and abscond with it to the US", as camoscato posted above. (Not literally the house, but certainly the loan. If they have no way to reach you in the US there isn't much they could do to chase you down for the money if you stop paying.)
- money laundering. If it were easy to get a mortgage on a property internationally, I would guess all sorts of people would just pour their illegal money into foreign mortgages. I'd be surprised if there weren't laws about who can obtain loans (of any size, but certainly large ones) when not a permanent resident/citizen.
It's actually discrimination in the US to deny someone a mortgage based upon their legal residency.
http://www.dhs.gov/files/statistics/publications/us-legal-permanent-residents-2011.shtmA legal permanent resident (LPR) or “green card” recipient is defined by immigration law as a person who has been granted lawful permanent residence in the United States. Permanent resident status confers certain rights and responsibilities. For example, LPRs may live and work permanently anywhere in the United States, own property, and attend public schools, colleges, and universities. They may also join certain branches of the Armed Force and apply to become U.S. citizens if they meet certain eligibility requirements.
http://portal.hud.gov/hudportal/HUD?src=/program_offices/housing/sfh/ref/sfhp2-07When the applicant has indicated that they are other than a US citizen on the URLA, the lender must determine residency status.
Permanent Resident Aliens are eligible for an FHA-insured mortgage under the same terms as US citizens.
Non-Permanent Resident Aliens are eligible for FHA-insured mortgages provided the property will be the borrower?s principal residence, they have a valid social security number, and the borrower is eligible to work in the US.
http://www.civilrights.org/fairhousing/laws/housing-discrimination.htmlHousing discrimination is a pervasive problem nationwide. It is also severely under-reported. The U.S. Department of Housing and Urban Development (HUD) estimates that more than two million instances of housing discrimination occur each year, but fewer than one percent are reported.
Many people are unaware that they have been victims of housing discrimination. A 2002 study by HUD suggests that many renters and homebuyers do not fully understand which activities are illegal under the Fair Housing Act.
If you think your rights to fair housing have been violated, help is available. Housing discrimination complaints can be filed by phone or in writing, with HUD and/or with private fair housing enforcement agencies located across the country.
The Fair Housing Act prohibits discrimination in housing on the basis of:
Race or color
National origin
Religion
Sex
Familial status (families with children)
Disability
Under the Fair Housing Act, the following activities are illegal:
Refuse to rent or sell housing
Refuse to negotiate for housing
Make housing unavailable
Set different terms, conditions, or privileges for sale or rental
Provide different housing services or facilities
Falsely deny that housing is available for inspection, sale or rental
For profit, persuade owners to sell or rent (blockbusting)
Deny any access to or membership in a facility or service (such as a multiple listing service) related to the sale of housing
Refuse to make reasonable accommodations in rules or services if necessary for a disabled person to use the housing
Refuse to allow a disabled person to make reasonable accommodations to his/her dwelling
Threaten or interfere with anyone making a fair housing complaint
Refuse to provide municipal services, property insurance or hazard insurance for dwellings, or providing such services or insurance differently