Why are there so many islands in the Pacific ocean, but not the Atlantic Ocean?

Many of the islands are volcanic, and the Pacific is much more volcanically active than the Atlantic. The reason for that, in turn, is that the central Atlantic is a divergent boundary between tectonic plates (the plates are moving apart, which produces spread-out, 'ooze-y' undersea lava vents). The boundaries around the Pacific are mostly convergent plate boundaries (the plates are pressing into the Pacific plate, either one going beneath the other and forming volcanos (Japan) or crumpling up against it a forming a mountain rage (the Andes).