I don't think you would. The split between topics allows sites to cultivate communities with strong domain knowledge. In fact, the whole process that sites go through to be created[1] relies on finding such a group to kick the site off. Many sites fail this process, becoming wastelands. Some are eventually shuttered. Those that make it to 'graduation' do so only because they have a community that helps create the content in the first place.
If the sites were merged, this sense of community and ownership would be lost. This is why each site is separate, with a style somewhat picked/influenced by the site's community, with ads chosen by the community, and rules drawn up by the community. Stack Exchange is the sum of many small (and a few big) communities, not one big one.