> Why can't a successful startup not be in Dallas, Amarillo, Miami or Des Moines?
Some of the burden is on you to prove what has failed so far. If it is possible to build a successful large scale startup in Amarillo, why hasn't it happened?
I'll point out the theory of agglomerative clustering also includes the contrary point: a city that is great at something tends to exclude other activities, for any given amount of population. A city like Amarillo, which plays a role in the oil industry, is less likely to be great at software, because it plays a role in the oil industry. It's population would have to grow before it could be good at both.
Also, if two 20-somethings build a successful startup in Amarillo, they may want to move to San Francisco as soon as they have money. The "this startup can be anywhere" argument works both ways: yes it could be Amarillo, but if it can be anywhere then it doesn't need to stay in Amarillo.
And the startup can also move to Poland, or Romania, or India. If you believe the startup can be anywhere, then you basically believe that a startup would only be in Amarillo by random chance. If there are 100,000 great cities in the world, and all of them equally good for a software startup, then the chance of a startup being in Amarillo is 1 in 100,000.
But in fact, there are good reasons why startups tend to cluster into certain cities. You should read "Why tiny Stockholm has the most stunning startup ecosystem since Tel Aviv" :
https://pando.com/2012/11/20/why-tiny-stockholm-has-the-most...