What's up with people not wanting to play a game because someone else has progressed further?

Recently playing terraria with friends. They were moving a bit slow so I started a single player world and finished it. When I went back to play with friends I used none of my new gear and only used what would be available. I had a bunch of extra summons so everyone could get stuff from the bosses we were killing and someone noted that I had extra stuff (the summons). When I told them I went and beat the game in single player, three of them were immediately uninterested in finishing and haven't joined back on. It's been a week and their excuse is that I've already explored everything and beat the game.

This isn't the first time this has happened. At one point in Pax Dei I did some extra mining and the player who wanted to be a blacksmith quit because I was also mining and giving them resources. Their stance was that I invalidated them. I don't get it, why wouldn't you want more people who can mine for rare resources? Why does me getting rare resources invalidate someone else mining?

I do not understand the mentality of people being discouraged by someone else's progression.

There was a time when a player "had the thing" it meant prestige and you wanted to be more like them, not throw your hands up because someone already had endgame armor before you.