The amount of xp you get is equal to your score in the match + game end modifiers (match ended and man of the match). Leaving a match before it ends makes no sense from XP perspective (as even 750 for a loss is most likely bigger than average personal score).
That said, there is no huge incentive to collect xp. It has no meaning in gameplay as even unranked matchmaking takes only MMR - matchmaking ranking, your hidden score - into account (unofficial source). The only winnable thing is the rank name.
Some people might choose to ignore getting XP (and good etiquette) in favour of being in the lobby 10-15 seconds faster.