Venmo significantly predates Square Cash, and once you get used to using a particular app for a task switching is a burden, especially for something like this with a powerful network effect.
The company was acquired, along with BrainTree, by Paypal to bolster their mobile and API-driven payments in the present age.
It didn't really matter that those mostly happened to people moving 5-6 figures, most of the millenials I know actively avoid paypal as a result.