Does Light Novel-based, slice of life anime in general have a clearly defined setting?
The Haruhi Suzumiya series is mainly set in Nishinomiya, Japan.

The Oreimo series is mainly set in Chiba, Japan.

It's quite obvious by the mentions in the Oregairu series novel and various episodes that the main setting of the series is based in Chiba, Japan as well.

Each author has their own reason for explicitly stating, excluding, or obscuring the main setting (just like the writers of the The Simpsons). There is no set rule in literature about whether or not the main setting needs to be explicitly stated, especially if it's fiction. A lot of fiction writers like to base their story on familiar settings, so that it's more easily related to readers (the Japanese audience being the primary readers) or give the author a easier time when they are world building. There is no trope in play, how much an author is willing to reveals about the setting of their story is up to them. Sometimes it's not the modern-day Japan as most would know it as, but such embellishment are implied and accepted by most readers of fiction.