Why can't I choose "Latest iOS" for the base SDK?

Why can't I choose "Latest iOS" for the base SDK?

In the project editor pane, with "Build Settings" selected, and with
either the project or the target selected, the choices for Base SDK don't
show any drop-down arrows. I can double click on the field and type any
text into the box but I remember that it used to have drop down choices
and the recommendation was to choose "Latest iOS". But now there are no
choices and apparently there is no way to enter Latest iOS. Is there some
configuration option I might have changed that caused the choices to be
removed?
What is in the box now is the text "iphoneos". By double clicking and
entering various strings I see that anything is valid if it begins with
iphoneos, no matter what text follows that. For example "iphoneosxyzxyz"
is valid. But if I enter any other text (for example "iphone" or "Latest
iOS") and try to build I get an error that says: "The scheme 'Metronome'
contains no buildables that can be built for the SDKs supported by the run
destination iPhone 6.1 Simulator. Make sure your targets all specify SDKs
that are supported by this version of Xcode."
(I'm using Apple's Metronome demo project for testing this problem.)
This is with Xcode 4.6.3.
I have another Mac also with Xcode 4.6.3 and if I run the same project on
it I see the drop down choices and I can choose Latest iOS and it builds
ok.
So the question is, did I change something to create this behavior? Or is
my Xcode damaged and I should reinstall it?