Because one week's worth of manga translates to about 7 minutes of screentime when animated. Adding in the opening credits and recaps padding things out, thats still only about 10 minutes.
Basically, 2 or 3 weeks worth of manga could fit into 1 week's worth of anime. When the manga takes 30 weeks to do 30 chapters, (and that's not counting breaks!) the anime would cover the exact same amount of material in just 10 weeks without filler. (And sometimes they mix and match. They'll pad episodes with filler, but then a random episode will contain two manga chapters, for pacing's sake.) So even being a year ahead isn't that huge a lead, really.
Also, on top of that, an anime episode takes more than 1 week to produce from scratch, it takes closer to 3 or 4 months to script, record, animate, and color, but they have multiple crews involved, which the only reason it can come out weekly. When an anime first starts, they very often wait until a manga has been out 2 or 3 years, and the early material is fairly free of filler, but once they begin to catch up, padding is necessary.
So for those two basic reasons.
1. A episode can fit more material than a manga chapter
2. A episode takes longer to MAKE than a manga chapter