(NewsNation) — The Biden administration had hoped to hammer out an agreement on managing migrant flows at the U.S. southern border during the Summit of the Americas this week, but the presidents of several leading migrant-sending countries will not be in attendance.

The presidents of Mexico, Guatemala, Honduras and El Salvador will not be attending the international summit meeting in Los Angeles, the first time the event has been scheduled on U.S. soil since 1994.

The president of Mexico said he wants no part in this summit because not every country in the Americas has been included.

Cuba, Venezuela and Nicaragua were not invited because their regimes have track records of not promoting democracy.

Leftist leaders have been critical of the U.S. decision to exclude the authoritarian governments.

Mexican President Andrés Manuel López Obrador said if they’re not invited, the summit isn’t worth attending and he’s sending a diplomat instead.

The summit is meant to bring together the heads of state and government of the Western Hemisphere to discuss common policy issues. A key component is to talk about regional migration and economic challenges seen in Central America and South America — a big part of the reason so many migrants make the journey to the U.S.

The absences are notable as the largest migrant caravan of the year is underway and heading toward the U.S.-Mexico border.

Several thousand migrants, largely composed of Venezuelans and Cubans. on Tuesday continued walking through southern Mexico while authorities showed no signs yet of trying to stop them.

Estimates vary, with some saying the caravan is composed of around 5,000 to 6,000 migrants, while activists on the ground peg the number at closer to 10,000.

Many of them are fleeing economic hardships in their home countries.

“In order to give our children and us a better quality of life,” Venezuelan migrant Yorgelis Díaz said. “It is awful to live in Venezuela. If we were doing well there, we would not have decided to leave our country like this, because this is hard, this trip is very hard, very hard, very hard.”

Inalia Mendoza, another migrant fleeing Venezuela said if the U.S. was more involved with her country this wouldn’t be happening.

“Look, we would be happy to return to our homeland, that’s what I long for the most: to return to my homeland,” Mendoza said.

The journey is a dangerous one — over 1,000 miles from Tapachula to the U.S.-Mexico border in the scorching heat. But very rarely do these caravans last long — typically they break apart before reaching the U.S.

“As they’re making their way up to the border, the cartels will break them up into small groups,” Brandon Judd, president of the National Border Patrol Council, said. “The entire number is still going to make it to the border, they’re just not gonna make it in one large group.”

This year there have been more than 776,000 migrant encounters at the southern border through the month of April.

The encounters break down by country as follows:

• Mexico: More than 300,000
• Cuba: 93,000
• Nicaragua: 53,000
• Venezuela: 30,000

Those four nations are the countries of origin for over 62% of all migrant encounters at the southern border this year.

The caravan provides a live illustration of the challenges governments face in managing immigration flows.

The influx of migrants arriving at the U.S. border has been a hot political topic all year, as the Biden administration has had to navigate a border surge that has seen a 20-year high in migrant encounters. Many, particularly Republicans and a handful of border Democrats, have criticized Biden’s handling of the border, saying his policies were too weak.

Others are calling on politicians to end their feud over border policy and pass real border reform.

“We have to have policy,” Judd said. “The taxpayer does not need to shoulder this burden. this can be solved through policy and policy alone.”

Despite the absences, the Biden administration says they’re still planning to roll out their migration agenda at this summit, and the president of Mexico says he will meet with President Joe Biden at the White House next month.

Vice President Kamala Harris did have a chance to meet with the presidents of Mexico, Guatemala and Honduras — the only three leaders she met in her two quick trips to the region.

During the summit, Harris is taking on the task of addressing the root causes of migration, for which progress has been slow. The region has seen little of her — a symptom, experts say, of larger U.S. neglect.

While the caravans have garnered media attention, the migrants traveling in them represent a small fraction of the migratory flow that carries people to the U.S. border every day, usually with the help of smugglers.

Mexico has tried to contain migrants to the south, far from the U.S. border. But many have grown frustrated there by the slow bureaucratic process to regularize their status and the lack of job opportunities to provide for their families.

Mexico’s asylum agency has been overwhelmed with requests in recent years as policies leave migrants few options other than to request asylum so they can travel freely. Last year, Mexico received more than 130,000 asylum requests, more than triple the year before. This year, requests are already running 20% above last year.

The Associated Press contributed to this report.