Most likely an IO limitation of the MediaTek MT7981B SoC used—it looks like it only has one lane of PCIe Gen 2, a USB 3.0 port, and one built-in 1 Gbps Ethernet controller (among a few other common low-speed interfaces)[1]. Typically these chips have bandwidth constraints, and it seems this one is meant for small routers, not the typical multi-2.5G or 10G setups some homelabbers would prefer.

[1] https://mirror2.openwrt.org/docs/MT7981B_Wi-Fi6_Platform_Dat...