Based on the table Encounter XP Multipliers from the Basic Rules' DM section, Page 57, when you're building encounters in 5E, you include an XP modifier depending on the number of creatures your PCs will be fighting: 1.5x for a pair, 2x for 3-6 and so on. However, the PCs only get the base XP as a reward.
This doesn't change the actual xp reward … just your calculations of how difficult the encounter is.
Why is this? After acknowledging that battles vs multiple opponents are more difficult, why not reward them as such?
Best Answer
A possible (and sufficient) motivation is to avoid encouraging players to game the difficulty system for greater experience gain.
A player aware of the XP system might be tempted to fight enemies in as large of a group as possible to ensure maximum XP gain. This would result in an over-difficult campaign and less fun for everyone, as the GM would have to do extra planning and the PCs would be consistently facing large groups.
A good way to eliminate this (and to encourage players to value high-level strategy as much as their PCs) is to only take encounter size into account when judging difficulty, not when judging encounter rewards.
This way you can have a kobold warren full of 20 kobolds in various chambers and not worry about the players deciding to kite the whole tribe into one encounter to maximize their XP. The players' desire for ease is no longer in conflict with their desire for advancement.