Per the documentation, Activity Map tracks buttons by default. But, why it isn't working for button on a page?
Analytics AppMeasurement doesn't treat buttons defined using <input> tags with type="button" or role="button" as actual buttons. Hence, these buttons are not tracked by default.
The solution is to remove the type and/or role attribute. Alternatively, Link Tracking can be implemented as recommended for non-clickable links or elements that are not tracked by default.