1. It's for game balance's sake. If you play an ECL race, you already lag behind 2 levels with levels, you don't want to have 2 less levels worth of starting equipment: For example, you start up with 3 levels. Your human friends have 2700 gm to spend, which could afford them a +1 weapon and a masterwork armor, whereas you, the party drow, only get a lousy rapier and a dirt cheap studded leather armor. I wouldn't like that.
2. As you go adventuring, everyone (usually) gets an equal share of the loot. So the 1st-level drow who travels with the 3rd-level humans would get the same amont as the others. When that drow's 8th-level and the humans 10th-level, they'll have accumulated the same amount of treasure.
3. Even though that drow has two less class levels, he's got the same amount of XP. That means that he's had the same amount of adventure as the others. And adventure means wealth. (He probably had adventures even before he reached his 1st class level. With 0XP, the Drow would just be drow, with 1000XP, he would maybe be 0-level apprentice, and upon reaching 3000XP, after quite a few opportunities to gain xp and money, he'd be 1st-level. Roughly as much as the human guy who's now 3rd-level)
4. Those ECL races are often mighty ones, and could have got that stuff when they started out as adventurers: that drow fighter won't be let loose on the world without a magical weapon.