Ain't it "easy" already?
Think about it this way: A game is a system of parts which define its function. For any game to exist, every part of the game must be created by someone. Creating just one part might be easy, but there are often thousands of parts required to make a complete game.
Engines are a reusable collection of parts. Someone realized that a lot of games all need the same subset of parts and decided to create those parts in a way that could be reused by multiple projects more easily. But the engine creators don't include parts that only one or two games need - those are up to you to create yourself.
How easy or hard it is depends on how many parts are already provided by the engine vs. those you need to create yourself.
Evaluating the difficulty of writing a game means figuring out what you have to do yourself. This requires knowing exactly what parts you need, and exactly what parts an engine already has. As the game creator, you're responsible for doing research into which engine(s) will minimize the extra work that you need to do. This itself is pretty hard to do; the level of detail you require to accurately evaluate this is greater than the detail provided by the feature summary of most engines. You have to read their API documentation to get a better idea, and even this isn't the complete story. You also need to actually try using the engine - sometimes documentation is out of date or things don't work in specific cases that the engine creator didn't anticipate.
Making a game is only "easy" if someone else has already done the work for you.