Because if you don't do that then you end up with inconsistencies across the stack because you used apps with different conventions. (Things like the help menu being in the wrong place, or a core app suddenly not having a menu bar at all because GNOME decided those are obsolete, or some applications ignoring the system theme, or OK/Cancel being in the wrong order...)

It's a complete ecosystem with design and functionality being consistent and on par.

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.

Technically, COSMIC uses libcosmic rather than iced. It is a custom toolkit based on a modified version of iced.

What do you think a desktop environment is? Name one desktop environment that does not have its own file manager or terminal. Name and shame the platform and toolkit that lacks these basic necessities.