Because some of us need bikes with long seat tubes and short top tubes, and they are not available. Add to that the need or desire to have a bar height near the same height as the saddle only compounds the problem. The answer to your question is that people can only buy the bikes in available sizes. Stems, bars, seat posts are made in a variety of sizes to get these canned-sized bikes closer to the custom size the rider needs. Put simply, NO bike fits very many riders; the rest of us make adjustments.