The reason is that things today are "fast enough." These days most slowdowns aren't the result of the CPU not executing instructions fast enough. Other factors dominate, such as memory access patterns, network delays, and interfacing with other complex software such as databases.
Unless you are doing compute-heavy code, the speed of the CPU isn't much of a factor in estimating how fast the program will run.
The reason is that things today are "fast enough." These days most slowdowns aren't the result of the CPU not executing instructions fast enough. Other factors dominate, such as memory access patterns, network delays, and interfacing with other complex software such as databases.
Unless you are doing compute-heavy code, the speed of the CPU isn't much of a factor in estimating how fast the program will run.