Why can a localiser be used without a glideslope, but not vice versa?
Adding to @J.Hougaard's practical reasons, there is one technical reason in the existing standards I can think of:

The identification signal (morse code) is provided by the localizer, not the G/S, so G/S-only approaches would be unidentifiable. (ICAO Annex 10 Vol 1 § 3.1.3.9)

Side note: I'm not sure if there can be a workaround for it using existing airborne equipment, since a G/S works on UHF (which is transparent to the pilot, i.e., the pilot only selects the LOC VHF frequency, and the G/S UHF frequency is simply paired).

The identification is likely one of the reasons behind the following:


  Inoperative localizer. When the localizer fails, an ILS approach is not authorized. (FAA AIM)


There is no mention of an operative G/S remaining. Whereas a failed G/S does not prohibit using the LOC for a LOC-only approach (after adjusting the minima).