Beyond these, the main reason is that gaming is a much better exploration and experimentation field than productivity because each app can have far less capital investment, lifecycles are short, and there's no need for a reliable, successful output.
Existing VR displays have your eyes fixed focused either several meters out, or at the horizon, so the kind of eye strain your talking about is not an issue.
MagicLeap supposedly has variable distance focus working.
Beyond these, the main reason is that gaming is a much better exploration and experimentation field than productivity because each app can have far less capital investment, lifecycles are short, and there's no need for a reliable, successful output.