Amazon lightSail provides simple un-managed VM instances, where you can do whatever you want. Lightsail is for small developers / SME that doesn't need complex functionality.
While, EC2 is a service which provides managed VM instances with Amazon "web service interface" which allows you to manage every aspect of your VM from single point. EC2 are meant for large organisations and SME's. EC2 are more secure, robust and easily scaleable. You can manage all your EC2 instances from EC2 interface.
There is no such "web service interface" with light sail VM's they are plain linux VM's with root access.