People who use GPLv3 want the code to stay open/libre under any circumstances. If this is the goal, why not use the AGPL instead, even for applications which are not served over a network?
This takes away the possibility that people integrate parts of your program into a proprietary network application, even if this seems improbable. There’s nothing to loose with using this license, but potentially some gain.
Only reason I can think of is that AGPL is less known and trusted which may harm adoption.
iirc richard stallman was too relaxed about it, saying it does not matter because “it is not your computer”, this mistake IMO caused the GPL to be unpopular.