In the case of the R5 the 4K60 ALL-I recording modes have a data rate of 940 Mbps (117 MB/s) in 8-bit or a data rate of 1000 Mbps (125 MB/s) when recording in 10-bit log. Since these data rates exceed the 90 MB/s minimum guaranteed write speed of V90 SD cards, it makes sense that CFexpress cards are required.Canon does the same thing with the R5, but even worse with just regular 4K/60p ALL-I. If Panasonic had a camera that utilized other media they'd do the same thing.
Now, whether such a high data rate is actually required for ALL-I is hard to say, but the 4K ALL-I recording modes on the R5 are pretty consistently 3x to 4x the data rate of the corresponding IPB mode (whether recording 4K30 or 4K60). The data rates are a bit on the high side, but for comparison, another ALL-I codec like 10-bit ProRes 422 at 4K60 has a data rate of 1257 Mbps. H.264 and H.265 are probably a bit more efficient than ProRes, but this comes more into play when there is temporal compression in interframe (IPB) recording rather than intraframe (ALL-I) encoding. So overall, these data rates don't seem so crazy.
The other thing to consider is that Canon does not sell their own CFexpress cards (unlike Sony), so I'm not sure that they have any real incentive to encourage people to buy particular types of cards. Not to mention that the CFexpress Type B variant that Canon went with in the R5 actually has a very competitive market with quite a few card vendors. And for the higher capacity CFexpress Type B cards (256GB or especially 512GB or higher), current pricing is around $1 or so per GB, which is actually fairly similar to the pricing for V90 SD cards of that capacity. So the higher capacity CFexpress Type B cards actually represent better value than SD cards since they are considerably faster, but comparably priced.
So, while the camera manufacturers are often guilty of more than their fair share of shenanigans, I actually think Canon's decisions are somewhat defensible here. Now Sony's requirement of CFexpress Type A on the other hand, well that's a different situation ...