Why didn't the Death Eaters use a house-elf to get into Hogwarts?

Just because you are a house elf does not mean you know Hogwarts inside out. The fandom wiki also says:

Apparition was by far the fastest way to get to a desired destination, but was tricky to pull off correctly and disastrous if botched.

Using elves meant might cause the Death Eaters to appear in random locations, and might require lots of round trips if they had more people than elves (I don't remember how many Death Eaters made their way to Hogwarts like that).

Whereas the vanishing cabinet in the Room of Requirement was guaranteed to work (after being repaired) and would place the Death Eaters in a known location within Hogwarts.

Strategy wise, if I had to invade a place in order to kill someone, I would choose the latter form of travel.

There are a lot of possible explanations:

• We don't know whether Kreacher apparated directly to Hogwarts in the scene you mention. He might have been apparating to Hogsmeade in order to walk to Hogwarts.

• There might not have been a house-elf available to them; Dobby had been freed, Kreacher was no longer available to the Death Eaters, and we don't know for certain that any of the other Death Eaters actually owned a house-elf.

• Draco in particular did not have access to Dobby any more, and as a pure-blood of a noble house, asking to borrow someone else's house-elf would have been humiliating to him. It would not be an option he would willingly consider unless he had no other alternative.

• Kreacher had Dumbledore's permission to enter and no ill intent towards Hogwarts. We can't be sure whether a house-elf would be able to apparate to Hogwarts under other conditions.

• Specifically, it is likely that there are magically-enforced laws preventing house-elves from using magic against wizards. That's why the Hogwarts house-elves did not use magic in the Battle of Hogwarts, and would presumably also prevent the use of house-elf apparition for aggressive purposes. (Dobby was able to use magic aggressively because he was a free elf.)

But all that said, if you asked JKR, the most likely answer would be that it was for the same reason Voldemort didn't realize that Kreacher would be able to escape the Horcrux cave - the Death Eaters consider house-elf magic to be beneath them, and simply aren't aware of what it can do. In particular, the idea that a house-elf might be able to do magic that they can't would be unthinkable to them.

A house-elf?

Those inferior beings that do your laundry?

It is beneath the dignity of a Dark Wizard to make an attack that depends on such a lowly being. It's as if your own magic were not enough, and worse, could be supplemented by such a creature.

No Death Eater would think of such a thing.