为啥I couldn’t care less.=I could care less.

这些都是与虚拟语气有关的用法.虚拟语气往往用来表达不可能存在或实现的事情或状况.
如果对虚拟语气有所了解的话,对这些句子的理解就会容易些.
我尽量做点说明,但语感还需要通过多读多练来培养.
I couldn't care less.意思是：我还可以关心得更少一点.（I could care less.） 要多不关心有多不关心.我一点也不关心.
I should be so lucky.我应该好运一点的,即事实上我不走运 （I’m unlucky.）
类似的用法还有：
I couldn't agree more.（我不能同意你更多） 我太同意你了.
I couldn't thank you more.（我无法对你再感激一点）我太感激你了.
I couldn't be less miserable.（我不能再悲惨了）我太悲惨了.
I couldn't have got to Slough in time unless I'd had a helicopter.
I couldn't have got to Slough in time if I'd not had a helicopter.
以上两个句子都是虚拟语气的过去时态.用过去时态应该是与上下文的语境有关（比如,全文可能是在讲以前发生的事.而这句话是讲在这个故事里某个时间点之前的情况）.如果去除过去时态,这两个句子应为：
I couldn't get to Slough in time unless I had a helicopter.除非我有直升机,否则我根本没法按时赶到Slough.
I couldn't get to Slough in time if I had not a helicopter.要是没有直升机,我根本没法按时赶到Slough.
这两句话的意思其实一样：我不可能按时赶到Slough,因为我没有直升机.
类似的用法：
I could build my own house if I had enough money.我要是有足够的钱,我就自己修个房子了.（我没有足够的钱,所以我不可能自己修个房子.）
I would take a vacation if I were you.如果我是你,我就休假去.（因为我不是你,我不能休假.）
当然,即使是同一句话,因为不同的上下文,其侧重表达的意思也是不一样的.
比如：
I would take a vacation if I were you.But I don't have so much time as you do.如果我是你,我就休假去.可是我没你那么多时间 （所以我不能休假）.
I would take a vacation if I were you.Why bury yourself in all those meaningless work?如果我是你,我就休假去.干嘛要把自己埋在那些无聊的工作里?（所以你应该去休假.）
不知有没有一点帮助?