Complete and likely useless speculation here, but:
I am fairly sure CFExpress uses SERDES 8b10b or 128b/130b encoding for AC line balancing. On some SERDES interfaces, idle characters must flow for skew sync and clock recovery which in this case would essentially be running the card when doing nothing. The data rate never changes even when idle and channel bonded. Perhaps there is a reason to stay in this idle mode at all times rather than disable interfaces to the cards completely. Also once hot, the cards are not going to cool just sitting there in a little toaster slot. Anyone an expert at the latest CFExpress application and physical layer? That would be actually a useful discussion since we know the cards in simple tests are part of the issue. In the end, even this will be speculating and just about useless compared to knowing the full details of the engineering compromises Canon was faced with.
A lot of Nikon users on various retailer reviews stated their CFE cards getting HOT in readers even with small transfers. I think CFE may have been necessary for the data rates needed, but a bad choice. Not sure if the SD slot is showing the same issues when doing 4K60 IPB. Mine overheated predictably, but did not seem to warm until placed in the sun and ran at high rate video. I don't even have CFE, readers or GPUs enough for 8K yet. 4K60 will be plenty for my needs. In most cases if I need slow mo, it doesn't have to be 4K or full frame, and I have several other products for that.
I'm fairly confident Canon can improve this situation, even more confident it was not deliberate (deliberate is a 30 minute recording limit). In the meantime, it is already a great camera for it's intended use, and the best hybrid on the market.