Canadian here. The majority who cross irregularly either withdraw their applications, or are denied refugee status. The large majority of applicants from certain countries (e.g. Nigeria or Mexico) are routinely denied. Refugee status is for people fleeing government persecution, not for poor people. The majority of the world is quite poor relative to Canada. Even if we were a far more generous people than we are in practice (quite stingy, really), we cannot realistically admit all of the people who want to move here because they come from impoverished countries.

Anyone can claim asylum in Canada. An American can try it if they want. Cross the border and present yourself to the nearest customs agent. You will get a government stipend of about $800 a month until you find a job, and some health care benefits... before being fast-track denied and deported.

Since we're a bureaucratic mess up here sometimes, it'll still take months. During which you can stay in Canada and get access to healthcare etc. And there's always the long-shot option that your claim of persecution, which is individually reviewed, might actually be accepted. If you can afford a plane ticket to the USA, and can cross over the Canadian border, then you can try this. Anyone can. That's what's being exploited.

There are some people fleeing persecution who would be refugees by Canadian law. We do accept quite a few of the border crossers. Chinese dissidents are a prominent source of refugees. Some people get desperate and will try anything anywhere they think they might be accepted. And Canada has a reputation as being accepting of refugees. An accurate one. Canadians are generally extremely pro-immigration, and we resettle more refugees permanently, than any other country in the world per capita. The government selects and brings tens of thousands of refugees from other parts of the world here, every year. But, bluntly, we prefer to do this on our terms.