Have faith. A planet with lots of dimethyl sulfide is bound to show up soon enough. There are a LOT of planets out there. The chances of earth being an anomaly are tinier than the amount of dimethyl sulfide on K2-18b.

Based on what I'm reading, we can't even really detect earth-sized planets yet. So if all or most of what we are measuring are planets that are substantially larger, we're not likely to find the good stuff when it comes to life (although I suspect there are some bizarre ocean worlds out there).