这是你自己误解了词典。你看看《韦氏高阶》的释义和例句，显然作为可数名词，用单数复数都是合理的。
3 [count] — used when discussing what is the best or most helpful thing for someone
It's in your (own) interest to keep silent.
The judge decided that it would be in the (best) interests of the child to continue living with her mother.
If he refuses to cooperate, he'll be acting against his own interests. [=he'll be acting in a way that is harmful to himself]
a policy that is not in the national/public interest [=that is more likely to harm than to help the nation/public]
The law is intended to protect/safeguard the interests of people who have no health insurance.
He claims that he has her (best) interests at heart. [=that he is concerned about her and wants to help her]
In the interest of [=for the sake of] fairness, we decided to get everyone's opinion before making a decision.
