As has previously been noted, DME are not azimuth aids and so they are not available in a navigation database as a navigable point. On a typical NDB/DME or VOR/DME installation the DME is very close to the NAVAID. In this case there are provisions made that these locations are equivalent and so GNSS in permitted in lieu of DME provided you are using the aid as the reference waypoint.
For a typical ILS installation the DME is located at or near the GP antenna, which is a significant distance away from the localiser array. The centre point of the localiser array is what is stored as the reference point for an ILS system, and so if you selected to navigate by that point there would be significant difference between DME reading and nav system distance. You will note on these charts that GNSS is permitted in lieu of DME but there is no reference waypoint given. This is the case where the Final Approach Fix has been named and there is reference to the threshold position. This has been done to enable navigation on an ILS using a coded ILS approach, where you can use the distance in the coded waypoints as a DME equivalent.
I hope this explains it....
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