[treeface]

For the same reason most companies don't share trade secrets.

- Stripe and Balanced ARE direct competitors, much more so than say Stripe and Braintree or Balanced and PayPal. Balanced handles the money in a bit of a different way, but the end product is very similar (i.e. receive payments from cards and bank accounts, and send payments to bank accounts [and now debit cards for both as well]).

- One or both companies may feel they have the better solution (see steveklabnik's comment above), and thus collaborating would be giving away intellectual capital for less in return

As a potential customer, you should prefer that they do work separately, because when different teams come up with different solutions, the chances are greater that at least one of those solutions is correct. This leads to greater long-term health in the industry as a whole.