Why is it “Who do you help?,” not “Whom do you help?”?

"Whom do you help?" is correct. But many English speakers use "who" wherever they should use "whom".

References:

https://en.oxforddictionaries.com/usage/who-or-whom

https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/

https://www.diffen.com/difference/Who_vs_Whom

https://www.dummies.com/education/language-arts/grammar/choosing-when-to-use-who-and-whom/

It is grammatically correct. In spoken English in Ireland and the UK it is actually over-correct and can sound either pedantic or ironic. In the U.S. and Australia, however, I've been told it is more commonly used in speech.

In a comment, John Lawler wrote:

They're both grammatical, but the use of whom has declined in modern English, to the point where substantial portions of the speech community actually follow different rules for its use because they use it so seldom. Since Anglophone schools teach their students nothing about English grammar (except what to avoid, for no reason anybody ever mentions), people have pretty much given it up as too much trouble. Consider: when you start a clause with whom, you're announcing that an object of some kind is coming up, though you haven't even given the verb yet. That takes a lot of processing.

First off, "whom do you help" is technically correct because 'who' is the object in that sentence, and 'who' gets inflected to 'whom' when it is the object.

However 'Whom' is an interesting word in the English language. It is a remnant from a time (old English probably) when nouns were inflected. English pronouns are the only system that retains this archaic rule, which is now non-productive, ie all new nouns coined are not inflected based on their role in the sentence.

Like others have said, it would sound pedantic or ironic in casual conversation. Personally I consider it obsolete in all informal conversations, and even most formal conversations. Others disagree and stick to grammar rules assiduously.