Snowden talked about this.

The US hold a big share of the electronic's intellectual property. Most of the tools used to design silicon chips are proprietary, and it would be trivial for the NSA to implant hardware backdoors without anybody knowing it, and it would be very difficult for the public to audit this hardware.

I think this is exactly why Huawei routers were not trustworthy.

Detecting malware is child's play, now if they want to not be detected, they need to target the hardware. Even if you know your hardware has a backdoor, it becomes too expensive or impossible to patch it.

Always wondered why wifi chips always used binary firmwares? Me too.