Getting a multiple entry visa for my wife?

I'm a USC. My wife is Burmese. We live in Singapore.

She has a multiple entry visa for the Canada.

When we visit the USA, we have to pay for a visa, and set up an interview. When she shows for the interview, they already have the visa approved. They always ask the same single question, "When will your husband move back to the USA?". They always give her a single entry. Why can they not just give her a multiple entry visa?

Next time should she tell them we're never planing to move back. We're only tourists in the USA?

Added (1). Sorry, typing on my iPhone.

Added (2). Why do they assume everyone wants to live in the USA? Many other countries offer a better standard of living, better law enforcement, and much lower taxes.

Because they don't believe that you ar never planning to move back. They do not believe that you are only tourists.

The question is partly a legal one: you are making a declaration of intent
so if you 'do' act fraudulently, and stay when you promised to not to.
Then any attempt to change status later (from within US) can be refused and they can even ban her for fraud - a silly game.

as to the question / single entry
You are a citizen of the US, and any one applying as the visitor of a citizen spouse, will be asked the same question (be it somewhere in Europe, or Canada even). As the Citizen has the right to settle, should you choose to do so, you could apply to bring your spouse - and also remain., A spouse visa application not a simpler (and considerably faster) visitor visa… And play the game to change later.

If you were to have her apply for a B-2, and attach a letter/statement to the effect that "you (as a couple) are settled outside the US, have not intention of returning to the US (as anything other than short term visitors) to settle. That should that position change, you are aware that you need the appropriate settlement visas. It would therefore be most welcome / be appreciated if a long term (10 year) visitor visa would be issued to support your and your spouses regular visits to see family remaining in the US."

That statement / attachment (like a company letter of invitation / statement of intent and support) would go a long way to sorting it.

I believe the problem is that Burmese are only issued single entry visas. Visa policy is
determined by restrictions placed on US citizens. If a US citizen can only obtain a single entry visa for Burma then Burmese are only issued single entry visas.