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.
Like with the gardening and cooking example above!
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)! :)
Learned a lot that gets you a paid position at an engine company that makes a lot more than 99.99% of indie games (probably makes more than a gameplay programmer too). "When in a gold rush, sell shovels".