The mathematical average is 1000, which is something taken from chess, where the Elo system originates.
If you set it to 0, you’d make it straight up unfair for new players as a theoratically good player can be matched up against a theoratically bad player.
As of now, you’re in the Top 50 % if your Elo is 1k. [(Source)](https://ratings.aoe2.se/)
Also, if a 1k player is matched up against a 2k player (rare but still can happen), the system asumes that the 2k player will win, so the reward for the winning player will be much lower.
Btw: the starting Elo was already lowered from 1600 to 1000 as former was/is used on MS Gaming Zone, Voobly and HD Edition.