Where is the fun in that?

You can always find coders asking why coding isn't fun anymore. I can somewhat relate but I never understood why the answer isn't obvious: coding isn't software engineering. When you go from coding to engineering, the focus changes. A lot of the interesting stuff is there, but there's also not-interesting-stuff in the mix. Maybe testing and documenting isn't your thing, you just want to build something. Maybe the stability from testing and documenting isn't that important to you. Perhaps you know you're the only one who's ever going to read your code. Your future self may be angry at you for a little while if the code breaks... so what? Your experiment crashed? Just reboot it. No problem.

If you're coding-to-sell, you're not writing code for yourself. You write for a team, even if that team is only you and future-you. You write it so it may scale and adapt to new requirements. You write it to survive a bit more than a weekend, and to be stable. You're not writing code to learn new things, that's only a nice side-effect; you are trying to build a product.

Furthermore, you're not investing time to learn something or just to have fun; you're trading time for money (if you learn something in the process, that's good - but probably not why you're being paid a salary as a software engineer).

It's understandable that parts of software engineering are not as fun as it was hacking in a basement while you were a kid. There is still a very big overlap, but it's not just the same activity. Myself, I try to focus on the fun parts and just have discipline to get the boring parts out of the way. I usually work in places where the balance is fairly decent, and it's kept me interested in software development for the last 15 (ish) years. I'm hoping it'll do the trick for much longer than that.