A) China's intelligence laws greatly suggest those blobs are going to have backdoors, particularly for the modem
B) Even if they weren't using backdoors, there's nothing stopping the US from unilaterally banning all Chinese made devices (something I definitely don't put past them since the TPLink incident)
It's already not guaranteed that the US govt won't outright ban the use of GrapheneOS, so adding the extra risk of Chinese manufactured phones getting the kibosh stateside makes me not want to risk $1600 on a phone.