It's the best data we have. When creating policy you don't get to use imaginary figures.
I feel like there's some confusion about what "military on the border" actually means. For decades we've had small numbers of uniformed military personnel on the border in various quantities performing duties with tools that civilian law enforcement may not have. This is common around the world and isn't particularly controversial. What some in this country advocate for is full militarization of the border with Mexico. Active duty, armor, fortifications you name it. This was brought up in HR2. The cost of this in Dollars and cost in an economic sense would be felt by every single person here. "Nearshoring" is the trend of the day with China seen as in increasingly unreliable partner and India being, well, India. Northern Mexico is seeing a huge amount of construction for factories to produce everything you can imagine. All of this is being constructed with the assumption it will not be in danger of becoming a conflict zone. Go back to what I said earlier about Mexico's reaction to having large numbers of American soldiers on their border. They would be well within their rights to also place their military up there for deterrence purposes. There doesn't need to be any open hostility between the two nations at all to throw a wrench in the economic development that's occuring on both sides of the border.
There's a sense among the Rs I've noticed that America can throw its weight around and tell everyone else what to do. Sure we could in a military sense but in very short order it would cost us economically. The world has changed and some haven't left the old Cold War mindset behind. The last thing we need as a country is for Latin America and South America to turn more towards China. It's now a viable option and we have to accept that. Ending the ridiculous number of asylum seekers and other border crossers depends on there being incentive for them to stay home and incentive for those here to go back home. It would cost us far less money to assist those nations in getting their economies in order than it would to try and stop them all coming indefinitely. We'd also ensure the Americas continue to be a conflict free hemisphere or as close to it as possible.