The question isn't why don't we see signs of other intelligent life? It is why don't we see signs of intelligent life within our species' technological lifetime?
Don't think of the universe as a sky full of points of light, where every civilization should be all shining at the same time. Instead I think of the universe as a vast grid of John Conway's Game of Life populated with a random scattering of 1s and 0s. Some of those patterns of cells will bloom and die out in a few generations leaving no trace. Some of those patterns will settle into small cycles but never grow into any complexity that we'd see. Some of patterns may grow into massive sprawling and seething tumults of cells for thousands of generations only to sputter and die before our cluster of cells is even born. Maybe some patterns will move across space like Life's gliders but the space is either so vast they never encounter anything or are immediately consumed by what they encounter.
Ultimately, we just might exist in conditions where the time and opportunity for civilizations to bump into each other (or even sense each other) is exceedingly rare. In some ways, that's a good thing, giving us time to develop before the unpredictable collision with other minds and cultures.