Sometimes it's a result of unforeseen consequences of design decisions.

All pre-signal Angular code must be compiled down to JS which replaces native async with Promise.

Why is that so? For a long time Angular's change detection worked by overriding native functions like setTimeout, addEventListener etc. to track these calls and react accordingly. `async` is a keyword, so it's not possible to override it like that.

Signals don't require such trickery and also allow to significantly decrease the surface area of change detection, but to take advantage of all of that one has to essentially rewrite the entire application.