Part of the reason why developers don't really focus in conversion optimization is that it requires continuous creative effort. It is not something like you do once and then be happy with it. Coming up with different variations which can perform well is hard and doing it consistently is even harder.
Another reason is that proper optimization requires a good amount of traffic, without which your A/B tests would never achieve statistical significance.
That said, optimization is pretty popular with online retailers and big web companies from whom even a 0.1% change in conversions/signups/etc can result into addition of tens of thousands, or even millions of dollars of revenues. For small-websites, the benefit of 0.1% may not be obvious. Hence, not much motivation for investing in optimization.
Another reason is that proper optimization requires a good amount of traffic, without which your A/B tests would never achieve statistical significance.
That said, optimization is pretty popular with online retailers and big web companies from whom even a 0.1% change in conversions/signups/etc can result into addition of tens of thousands, or even millions of dollars of revenues. For small-websites, the benefit of 0.1% may not be obvious. Hence, not much motivation for investing in optimization.