Hi all – I’m the codeowner of simplisafe-python and the SimpliSafe integration in HASS. 
As you can see from simplisafe-python, it’s possible to get sensor values via the undocumented, private REST API. The challenge is that these API calls aren’t local; they go through api.simplisafe.com. Considering that things like door sensors are only useful if they provide near-real-time data, I’d have to query the API once every 1-5-10 seconds. Simply put, because we are using an API that’s undocumented, I’m concerned that slamming SimpliSafe with thousands of new API calls every day will compel them to block access to the API altogether. Therefore, I’ve elected to refrain from implementing sensor functionality for now.
Believe me, this is my white whale – I’m constantly on the lookout for new, more appropriate methods of interacting with the API. Until SimpliSafe changes from their current model (in which (a) we interact with their cloud API and (b) that API only provides updated data when the connection to the base station is refreshed on-demand), we’re stuck.