JDJblatherings said:
Because we think we need to or we do need to have those relationships.
But D&D players, by and large, do not need to or think they need to have their PCs have those relationships. By building up the idea that "All NPCs Are NOT Out To Get You" the DM has the chance to have the players actually
enjoy their relationships with NPCs.
Examples:
PCs help a merchant recover his son, who foolishly went adventuring in a dungeon. Later, that merchant gives the PCs a letter of reference, and aids them to secure cheaper passage downriver.
The PCs decide not to kill an orc, but simply let him go. They later meet that same orc, who now respects the PCs.
The PCs rescue some prisoners, including one who shouldn't normally be trusted (an orc), and one who is secretly going to betray them. When the betrayal occurs, the orc NPC sides with the PCs, effectively giving the PCs the edge they need to go from a difficult encounter to something that is merely a blip on the radar. (The PCs later heal the orc NPC instead of one of their own when he is injured!

)
NPC farmers allow the PCs to stay in their home, offering to sleep in the barn themselves (the PCs instead sleep in the barn). NPCs frequently buy PCs drinks or share their meager dinners.
In short, NPC betrayals, and threats to NPCs, are more useful when they are
unusual interactions. Give the players reason to care about the NPCs, and it is more likely that they will care.
RC