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.
Both GPL and AGPL are open software liceneses which create an obligation on people who “publish” the binary. (“Publish” is a term of art here.) The difference is when the obligation is triggered: In the case of AGPL it adds the obligation when a modified version is used over a network to provide the source code of the modified version running there to the users of that server. Therefore, the mere public use of a modified version, on a publicly accessible server, gives the public access to the source code of the modified version of the AGPL licensed software.