Because if they do, it’s almost pointless upgrading Supply Depots. At least, it makes me a lot less willing to invest in them. I’ve been doing more WvW and only using money I earn in WvW. So far I’ve noticed it’s do-able to only upgrade (at least tier 1 and 2 upgrades) using this method, but haven’t gotten farther than that.
Because if they reset so easily, then for the people who are actually investing, WvW could become a major money sink.