Well, it's two different use cases. PCs are platforms, and they have common components and optional peripherals. Embedded systems are completely custom designs that rarely have common components. Everything is driven by BOM cost, and if you don't need something, you don't add it.

Booting such a system is highly dependent upon the board configuration and the peripherals that are needed. Furthermore, vendors have different boot-up requirements. Some boot loaders perform proprietary system checks before loading the RTOS. Some boot loaders need to be able to upgrade firmware from an image on-chip, in flash, or fed into it over SPI or a UART. Some boot loaders are little more than a jump table decoder that just jumps to the currently active firmware image on the chip. Creating a universal boot loader with enough flexibility for each of these situations is harder than just implementing a boot loader for a particular electronics product or product family.