I have no idea how hard it is to add or not, but people have been requesting this since the first day that smart home monitor was released almost 2 years ago. 
In fact, at one point over a year ago the CEO of the company was posting to the forums and asked people to explain what an exit delay was. Which they did, but it didn’t make any difference.
So right now, you can file this one along with official support for a keypad, The ability to arm/disarm without access to the cloud account, the ability to dismiss multiple notifications at once, Cellular connection, the ability to add a secondary user who doesn’t have full admin privileges, the ability to use the app without going through the cloud account, The ability to create sensor zones, the ability to remove access for a secondary user who hasn’t logged out of their mobile app, additional notification options including SMS notifications to more than one number, ability to flash lights as a notification, ability to choose siren or strobe on devices with that capability, ability to customize the dashboard.
Those are just the basics, there are additional features that some people would like to see. But these basics are included in most purpose-built security systems in the low-end price range.
So it just seems pretty clear that whatever SmartThings is intended to be, it’s not being built to really compete as a home security system. Which you can tell by reading their official usage guidelines:
https://www.smartthings.com/guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.
Definitely feel free to keep requesting additional features. But it’s not like people haven’t done that already.
Just sayin’…