Purely due to random chance that a tree branch grew too weak and fell at the wrong time.
You still need shade, and so do others, and if you're doing any business that requires shade, you have no choice, but these 3 trees.
Even in a narrower scope, just talking about mass market businesses selling consumer products, an example would be the huge number of Walmart suppliers that do fine without sitting under those specific '3 trees'.
Yes a large fraction do sit under the big 'Walmart tree' but they're not chained to it forever, the big 'Costco tree' is also available, along with many other smaller trees.
But even then, in a very interesting twist of absurdity that is the current state of affairs, your argument gets even more funny when you consider that Walmart spends some 4 billion dollars on ads per year, and are one of the largest customers of Google ads and Facebook Ads. So even Walmart depends on those 3 'trees'.