What you are describing is not equanimity, because there is an attachment to not working/relaxing. When you ask why not just lie down and die if you are content with everything, you could just as easily ask why not work really hard if you are content with everything? Because if you really have equanimity, working hard would cause no less contentment and no less suffering.

A big part of mindfulness is narrowing down your reality to only this moment, because that is really all that's real. The past isn't real and the future isn't real. It's not about accepting "your life" because that's not actually a real thing. Right now is all that is. All context is actually fully contained in this moment. So one can accept and be at peace with what is happening right now and see it for what it is, then, thus informed, make and follow a plan. But each moment has it own independent meaning and is an ending in itself. There is no final goal because it's always right now, so the rational approach is to always pay attention to what IS.