Q:

Why is "that"/"which" omitted in this sentence?

In the sentence:

All the information you need is in these folders.

I would have expected to find:

All the information that/which you need is in these folders.

Why are those omitted?

A:

A relative clause whose referent is the direct object of its verb does not require an explicit relative pronoun at its head. 
Traditional grammar calls this ‘omitting’ the pronoun, as Carlo_R does. Recent academic studies say rather that the  pronoun employed is the ‘null-relativizer’, symbolized with Ø; I don’t know why this terminology is preferred, but the symbol is certainly convenient for calling attention to the absence, as below.

 All the information Ø you need is in these folders.

As Carlo_R points out, in formal discourse this omission (or null-inclusion) is not permitted when the relative pronoun is the subject of the relative clause; but you will encounter it frequently in speech:

✲Bernard Shaw, Ø was an Irishman, is regarded by most critics as the greatest English-language dramatist since Shakespeare.  BUT 
 You know the guy Ø runs the shoe-repair place on Dorsett?
   I was sitting in the Union and that girl Ø sits in the front row of our Shakespeare class came up to me.  

In formal discourse the omission is also forbidden when the relative clause is non-restrictive, even if the relative pronoun is direct object; but, again, this prohibition is ignored in speech:  

✲Bernard Shaw, Ø Lawrence admired greatly, actually proofread The Seven Pillars of Wisdom.  BUT 
 Sharon, Ø you just hit on, ’s the wife of the precinct captain! You’re in deep doo-doo, man.

The last sentence would probably be spoken with a rising intonation on Sharon and hit on, as if it were punctuated with question marks?

 Sharon? Ø you just hit on? ’s the wife of the precinct captain! 

✲ marks an utterance as unacceptable

A:

Complementizers (words which introduce an embedded clause) can be elided in English.
So the answer to your question is: it is omitted simply because it can be omitted, and the writer and speaker chose to do that.
Sometimes speakers and writers take advantage of this and save a word, and sometimes they put in the complementizer that or which, even though it isn't necessary.
Complementizers can be omitted even in sentences like these: "Tell Bob Jones stopped by". Although at first glance it looks as if "Bob Jones" is one name, it doesn't make sense that way. Furthermore, in speech, it can be made clear where the break is.

