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.
@rbn@feddit.ch made a well structured comment using bullet points. It can be read pretty quickly but I guess the tldr is the last line:
if you want to know why you have to spend 20 seconds reading the preceding text.