Because muscle building isn't just micro tears and repairs of muscle fibers. Actually, it's not totally clear that those are directly correlated with muscle growth; some lifts, like the deadlift, the Olympic lifts, box squats, and partial squats or presses done from pins or a box, have either a vastly reduced eccentric component or none at all, and therefore they do not cause significant microtrauma; however, they nonetheless lead to increased size and strength. Microtrauma has also been shown to decrease with training; hence why your first workout after a layoff will produce unimagined levels of soreness, but your tenth or eleventh will produce almost none. My understanding is that satellite cells are able to contribute to the growth of muscle cells even if there is no microtrauma.
The actual process by which the body "figures out" that new muscle tissue is required isn't fully understood. A great deal of it definitely has to do with the flood of anabolic hormones post-workout. Strength gains also come from not just the synthesis of new muscle tissue but also from increased neuromuscular efficiency; likewise, size comes not just from myofibrillar hypertrophy, wherein the actual muscle fibers increase in diameter, but also from sarcoplasmic hypertrophy, wherein the muscles basically store more fluid. The continuum here explains why Olympic weightlifters can be very explosive but not particularly large and why bodybuilders can be very large but not particularly strong.