The fact it is a Chinese company is all the evidence needed. Huawei is subject to China's 2015 National Security Law, which grants the Chinese government the power to force Chinese companies to provide assistance in matters of national security. In other words, if Chinese intelligence wanted Huawei to secretly introduce a backdoor, it would be a legal order and unlawful for Huawei to resist. What makes you think the US needs to have evidence beyond that when this impacts critical infrastructure and national security? Why take the risk?