I guess you don't have much experience with cuevas. I do. (And I know how Ambito supposedly gets its information, the survey of cuevas. Do you have any idea how many cuevas there are? How many cuevas do you think Ambito surveys? And how do you think that all the newspapers have the same prices within minutes of each other, even when the prices are changing rapidly? These past few days the blue price was changing sometimes five times or more in an hour. Think about it.)
The other day I went to the cueva I use most. He was offering 4 pesos less than Ambito was showing. His price 422, Ambito 426. So I said I'd return, and I walked around Florida and asked prices. I was offered prices of 415, 410, 412 and 416. They all insisted it was their best price, even when I showed them Ambito's price. "No, no, señor. Imposible." I went back to my cueva and got 423.
My apologies for the length, but I think you're confused..
You're dealing with street corners arbolitos & Calle Florida which is notoriously known for being shady. These people have 0 police protection & you're taking a large risk by buying USD from them as they don't stamp.
A legit cueva in zona norte, palermo, etc has employees, rent, expenses, etc to pay. They also routinely lose money on devaluations. To compensate, they charge a commission. It's the same reason you will never get the central bank USD/EUR rate, at minimum it's a 1.5% commission from places like visa, but normally much higher. It's not possible to operate offices on a 0.75% ambito buy/sell spread.
Ambito, cronista, etc are contacting the "central cuevas". Who they are is not public information but it's probably people inside the central bank. The "central cuevas" provide liquidity to the smaller offices, when someone buys 40 or 50k usd after selling a car or whatnot, now the cueva has backpacks full of pesos they need to immediately liquidate to USD what they won't sell that day - they go to a "central cueva".
In Argentina, the dollar blue is a government backed institution, a "cueva" is a bank & a legit one should have the same security. My "cueva" was granted essential business license & never shut down during covid (entire calle florida was raided by police during covid...), they can arrange to safely transport large amounts of cash, every thief in the city knows if you touch their clients they're going to get rained on & they stamp all USD.
As I said before, you can find people who will give you better rates (if you really want the best, go deal with smugglers in border towns, ambito+10-15% is normal there), but these people are not professional money exchangers & they profit in other ways.