@Pesala To me it's more of a principle. I don't see the sense of saving something I don't want or need.
Mod Edit: Typo corrected. To edit your own posts, use the three vertical dots icon at bottom right of each post.
@Hioma I think it is to protect careless users who close Vivaldi when they did not intend to. Recovering closed tabs from the trash can is easy, and if you want to clear them from trash it is also easy — just press the Access Key shortcut (C for English UI).