I wonder why devs need to work on multiple different environments. I only have (user)experience with KDE and GNOME but what’s the difference between them?

• It’s not the default layout, you could ship another default version
• goals can be to maximize customizability (features) while minimizing ressources. Or the other way round. (Or simplicity)
• is it because devs like different frameworks (qt, gtk)? Or language?