It's just not been implemented. It needs a separate mechanism on mobile, because click-and-drag doesn't apply with a touch screen, and touch-and-drag is already used for scrolling.
On mobile there's actually a range of features that aren't supported, including entire major features like the Timeline Bar, because making them work on a small screen with a touch interface would be a major project. That's why we say the mobile UI is designed for reviewing and tweaking projects, rather than main development, which should be done with the desktop UI. On larger devices like tablets you can put Construct in to desktop mode in Settings, attach a keyboard and mouse, and use it like a small laptop.