You put out a welcome mat and people come to your country, especially if your country is richer than theirs. Any professions of surprise from Democrats about the immigration wave are either disingenuous or abjectly stupid. From the Issues and Insights Editorial board at issuesinsights.com:

Just one week ago, Department of Homeland Security Secretary Alejandro Mayorkas flat out denied that there is a crisis at the southern border. “The answer is no,” he said in answer to a reporter’s question. “I think there is a challenge at the border that we are managing.”
The very next day, administration officials informed President Joe Biden that the number of migrant children in the U.S. is on pace to beat the previous record by 45% and that there aren’t enough beds to accommodate them.
Then Monday, Mayorkas started pleading for volunteers to help Border Protection cope with the “overwhelming” surge of illegal border crossers. “Your colleagues need assistance,” he said.
In January alone, Border Patrol agents had 78,323 “encounters” with illegal border crossers – which is higher than any January in a decade. In the three months since Biden won the election, these “encounters” totaled 224,337, which is 87% higher than the same three months a year ago.
According to the New York Times, there were likely 100,000 more “encounters” in February.
NBC News says that a “record number of unaccompanied migrant children” are being held in “facilities meant for adults” and nearly half have been held for longer than the three-day limit.
“The CBP holding cells, sometimes known as ‘hieleras,’ or iceboxes, are not designed for children. They are typically small concrete rooms with concrete or metal benches and no beds,” it reported.
Yet Biden won’t explain why the surge is happening, much less take responsibility for it. He won’t even admit there’s a problem.
Last week, a reporter asked Biden: “Did you receive a briefing about the border today?”
Biden: “Uhhhh. Yes, I did.”
Reporter: “What did you learn?”
Biden: “A lot.”
Then Biden left the room.