I tried your code as a .vbs script and excel closed for me.

In fact, I thought maybe there was a prompt waiting for you so the xlwkb.close
line was still waiting.

If you have a volatile function (like =today(), =now(), =rand()), then that
workbook will recalculate when you open it. And maybe excel was waiting for you
to answer the "The workbook has changed--do you want to save it?" prompt.

But that message popped up nice and clear when I tested. (You would have
noticed it.)

Maybe adding

xlapp.visible = true

to your script would help you find the problem.

(tested with winxp home and xl2003)