Learn Dutch: We moeten de tekst voorbereiden.

Because of the modal verb moeten (must). When a separable verb is the main verb in a sentence with a modal verb, the separable verb stays glued together as one word in its full infinitive form and goes to the very end of the sentence.

Why does voorbereiden go at the end instead of right after moeten?

In Dutch, a conjugated modal verb like moeten takes the normal second position in the sentence, but it pushes the main action verb (voorbereiden) all the way to the end. That is why you cannot say "We moeten voorbereiden de tekst".

How would this sentence look if we didn't use the modal verb moeten?

If you just wanted to say "We prepare the text" without "must", the verb would separate. The main part would go in the second position, and the prefix would get pushed to the end: We bereiden de tekst voor.