Git is expected(though not required) to be installed on the client machine where you are running "oc new-app". No one should be using an rhcos machine as their client machine for invoking oc new-app, so installing git on it is inappropriate. oc new-app is a developer facing tool.
I completely agree with Ben here. Once our images move to UBI, this will partially help w/ repos, but it will only have a subset of content and I don't believe git is included. As a workaround, toolbox can use the rhel-tools container. create ~/.toolboxrc REGISTRY=registry.redhat.io IMAGE=rhel7/rhel-tools:latest