The main reason is that some mechanics are severely nerfed when the game is NOT set to hard mode. For example, mobs are less likely to pick up an item you drop (to keep them from despawning), and raids have fewer waves.
In an earlier time, the "Normal" server we play on currently used to be called Semi-Vanilla Survival. Normal in this sense resembles more of a "close as reasonably possible to vanilla minecraft" definition over a difficulty definition. All sandlot survival servers have this definition in some sense. Easy - has a job system to earn money, making it quicker to get harder to find blocks and items. Normal - Minimal assistance from added server plugins. (a couple tweaks like teleporting to spawn, a home command, etc.) everything is mined/farmed by players. nothing arbitrarily brought in. Closest server to vanilla minecraft. Hard - Changes one or more minecraft game mechanics to provide a different challenge to the usual experience. Usually resets every month with a new twist. Skyblock - A mix between Easy and Normal as far as vanillaness, but everyone has their own island. If I recall correctly, all servers have the difficulty set at hard, because all minecraft mechanics are available on that difficulty setting.
I still call it Semi Vanilla sometimes haha But I kind of think we should bring that name back because it just makes more sense. Hard is more like Ultra Hardcore (which I believe it used to be called?) I understand Easy, Normal, Hard is much easier to remember but I liked the cool old names!
I think Easy has the resource world set to normal levels, but the overworld is just a bit easier dealing with mobs.
