Partly because people replace phones more often than computers. But also the RAM they use in portable devices was made specifically for the device and integrated directly into the mainboard. There’s less points of failure and compatibility is never an issue. Since desktop RAM can be replaced and upgraded, it’s not as big of a deal if it fails, you can just swap it out with a new stick. Whereas it would brick the whole device if mobile ram fails, so quality standards are much higher.