but I'm wondering why in UWord project, richtext compiled output can be static lib, however richedit cannot?
Uh, in standard U++/theide terms, it is irrelevant... You have project and you have executable. The process is meaningless.
Now if you want to avoid theide, packages etc... and want U++ as library, that is different thing. Search through the forum, there is quite a lot of debate about that here.