You don't get it.
Building a game engine and building games are two separate hobbies.
It would be like gardening and cooking. You may like both, or just one, or none.
When you make yet another toy game engine, you've hopefully learned a lot, but what you have is a lot of dead code.
You might be able to build a perfectly wonderful garden, but that doesn't mean you can use the vegetables you carefully cultivated in any meaningful way in your kitchen to cook a healthy and delicious meal if you can't cook at all (but you can still share the vegetables with your neighbors who might share their cooking with you)! :)