For first year(s?), Broadcom RPi components (GPU) were closed source (opaque binary blob) and need RE work. For a hardware supposed to be "open to hack", it was considered a non-sense. I suspect things have changed regarding RPi situation.
Intel has ME/AMT, and requires an opaque, PK-signed-and-only-intel-and-NSA-have-the-key blob. AMD is the same with PSP.
I'm not up to date, there might be a RISC-V core available today that is blob free, but in 2012 when RasPi was introduced I wasn't aware of anything blobless.
But why do you trust that the silicon doesn't have any backdoors more than you trust the blob? We know for a fact that Intel puts one into the silicon, and that it takes significant reverse engineering to turn most of it off (it's not clear if all of it can be turned off at all) see e.g. https://hackaday.com/2020/06/16/disable-intels-backdoor-on-m...
https://forums.puri.sm/t/the-i-mx8-cannot-be-deblobbed-nxp-s...