Because at some point you will pass the point of diminishing returns. Suppose 3% of the time is spent in the server, and 97% in the web app. Do you really want to spend 200 hours trying to reduce that 3% to 2.5% instead of spending 10 hours on making your app twice as fast? You've made the server 20% faster, but what have you really gained in practice when you look at the overall picture?

Even assuming unlimited developer resources, will you really notice a 0.5% difference in production, especially when taking things like network latency into account?

My point is that hyping over raw performance is misleading at best. If performance is the only thing that matters then we wouldn't have operating systems with abstractions - everybody would program against the bare metal in assembly. One should weight the pros and cons carefully and reach a balanced trade-off.