like newt already alluded to:
The way Unreal and Unity can be free is that they have a few huge AAA games made in the engine that pay millions in licensing fees (the fees are based on the games revenue as far as I know).
With an engine like Construct 3 that (from my point of view) is more so targeted at beginners and small indies, there probably isn't as large of a revenue generated from the top games for a similar system to work.