Why not? Right now I have my nsec in app A. You want me to download another app (app B) and put my nsec in there. Doesn't it make sense for me to object and say I just want to keep it in app A alone? If not, why not?
Discussion
Because a signer app do more, and you do not have to implement all this functionality in all apps
You can choose to run Amber completely offline with no OS permissions whatsoever.
Not the case for Amethyst.
It's like moving your passwords from chrome to keepass.
it makes no sense that every app has to implement dm-signing
it makes no sense that just because you happened to have pasted your nsec in one app in the past that now that app has to be the app that will be used as the master signer guardian of nsecs forever
if your ideal is that people won't input their nsecs in random apps but will keep it in only one then ideally also amethyst would stop even accepting nsecs, relying only on amber, and you should support that