Because the guests were not staying exactly in Hogwarts, they were staying in the vessels they arrived in.
Durmstrang in their ship and Beauxbatons in their carriage.
They were 'parked' outside of Hogwarts, so the carriages were there to take the students from their vessels to Hogwarts.
After a lot of research online to find an answer, I have a theory for this now - The intention of making Dumbledore offer these magical, strange, semi-sentient, toothed sweets to Harry is nothing but to show the humorous side of Dumbledore's character. The film-makers have done this at a few places in the entire series and sometimes (like in this scene) they ...