treat others the way you want to be treated为什么不是treat others in the way you want to be treated

你的理解是正确的,the way, 做后置定语,介词in省略了

因为这是个补语从句，you want to be treated 是修饰补充the way
treat others the way THAT you want to be treated

the way 可做做连词

你的理解也是对的，但是the way还可以作为连词用哦，我在网上帮你查了些它连词用法的介绍，具体如下，希望能帮到你：
“the way”作连词用法小议
1、 the way 相当于how，作关系副词，可用来引导宾语从句或表语从句
Eg.1. I don’t like the way you speak to your mother. 我不喜欢你和你母亲讲话的方式。
此处the way充当连接副词，相当于how 。 the way you speak to your mother就相当于how you speak to your mother，作don’t like 的宾语从句。而我们知道，引导宾语从句的关系副词how=the way in which。因此，理解the way 与you speak to your mother 之间的关系时，您可以认为后者是前者的定语，译为你和你母亲讲话的方式。the way 是先行词，介词+关系代词（in which）的引导结构这里被省略了，you speak to your mother是定语从句，完整的结构应为the way in which you speak to your mother 。
类似的用法有很多：
Eg.2. I'd like to know the way you solved the problem. 我想知道你是怎样解决这个难题的。此句相当于(I'd like to know how you solved the problem.) 。
Eg.3. That's the way she did it. 那就是她是怎样做的这件事。相当于(That's how she did it.)。 其中how she did it 做了表语从句。
2、the way 相当于 as，作连词用，引导方式状语从句，表示“像……一样”
Eg.1. They didn’t do it the way we do now. 那时他们不像我们现在这样行事。（相当于They didn’t do it as we do now.）
Eg.2. You must do it the way I told you. 你必须像我说的那样去做。(相当于You must do it as I told you.)
Eg.3. I won't talk to him the way my father did. 我不会像我父亲那样跟他讲话。(相当于I won't talk to him as my father did.)
2、 the way 用来代替in the way in which 时，the way 常起连接词作用，引导定语从句
Eg.1. I fixed things the way (= as) he wanted. 我照他所要求的方式把事情做好。
Eg.2. What Surprised me was not what he said but the way he said it. 令我惊奇的不是他所说的话，而是他说话的方式。