It's really necessary to start thinking in terms of the total width of the memory bus. DIMMs have a 64-bit wide connection. Dual-channel used to only mean half of your DIMM slots were on channel A and half of them were on channel B, for a total of a 128-bit wide memory bus if you populated both channels.
DDR5 splits the DIMM's 64-bit connection into two sub-channels of 32 bits each. But mainstream CPUs still have the same 128-bit total width for their DRAM controllers, so you still need more than one DIMM installed to use all the (sub)channels provided by the CPU's memory controller.
Comparing the 128-bit wide memory bus against the bus widths used by discrete GPUs (and comparing the memory clock speeds) makes it much less surprising that running integrated graphics in a crippled 64-bit wide memory configuration would be problematic. Graphics is a very bandwidth-hungry task.