Because the are not using an existing toolkit like GTK or QT. They use Iced, a pretty new toolkit. Developing applications helps them making that toolkit fully

Because the are not using an existing toolkit like GTK or QT. They use Iced, a pretty new toolkit. Developing applications helps them making that toolkit fully production ready. The can reuse the same libraries and components across lots of applications, terminal, editor, settings and so on. Look at 'cosmic-text' for example. Plus it makes sense since apps developed with GTK/QT will not show of the full advantage of the new DE. System76 has to support everything they shit to costumers. So if they ship Alacritty and it has a bug they need to fix it. So it makes sense to have tight set of libraries and application that they control.