You can start from any run on any device, though. You can scroll the "progress path" at the bottom of the Get Running home screen from left to right and tap on any run. Once you've tapped it, it'll highlight in blue and when you press "Run" Get Running will start off from that run. So for a one-off run on a different device, that can work okay.
We've occasionally been asked for syncing functionality, but the vast majority of people seem to use just the one device for their runs, and we have to balance demand against the increased complexity of synchronising data, maybe having user accounts and passwords, synchronising between Android and iOS, and so on.
(There's a bit of a privacy issue here, too. At the moment we collect no data at all about our customers (unless they email us or sign up to the forum!) and we're pretty happy about that. No information about you or your running ever leaves the phone, unless you choose to deliberately share details of a run on social media. A sync system would need to break that wall of privacy to some degree...)
Hope this explains the current situation!