I think there are two reasons: First, SBC processors are just a step up from microcontrollers, they're designed to talk to GPIO devices and servos and UARTs an

I think there are two reasons: First, SBC processors are just a step up from microcontrollers, they're designed to talk to GPIO devices and servos and UARTs and sensors, not GPUs and network cards. The JH7110 only has a two lanes of PCIe 2.0, one of which is used for USB 3.0 and the other mostly (I assume) to provide an M.2 interface. However, it also has 6 UARTs (Universal Asynchronous Receiver-Transmitter, think RS232 serial), 7 channels of SPI (not counting the QSPI Flash controller directly integrated), 8 channels of PWM, 7 channels of I2C, 64 channels of general-purpose IO, I2S, SPDIF, two channels of CANbus, a directly integrated Ethernet MAC, USB 2.0 host, a MIPI-CSI camera interface and a MIPI-DSI video display block with either HDMI output or for directly driving a 24-channel parallel LCD. Embedded system designers don't want to add a PCIe-to-RS232 card to their industrial robot, NAS, or video camera, heck, they don't want to add an external GPU. They don't even want to add a separate northbridge/southbridge or PCH, they want a single-chip SOM. Going up and down those layers between PCIe and SATA or USB or Ethernet is expensive in terms of chip count and power. Second, I don't think they want to deal with the drivers. If you want to plug in your choice of PCIe device - be it a GPU, RAID card, sound card, who knows what - that's a level of compatibility and modularity that SBCs are bad at.