There are three critical components to consider.
One: portkeys are not supposed to be used to transport oneself onto or out of the grounds of Hogwarts without prior knowledge of the ministry and Dumbledor. It is possible to break this rule (as we see Dumbledore do himself in Order), but as the only example of a wizard doing this without ministry permission is Dumbledore himself, Crouch probably wasn't capable of it. The cup was an "allowed" portkey and was supposed to transport the winner out of the maze and back to the stands to be celebrated. Crouch Jr. altered it a little before placing it by adding the portus to the graveyard. This is simpler than layers of spells probably needed for circumventing portkeys that have not been "registered" so to speak.
Two: While Crouch could have killed Harry and then escaped, Voldemort wanted to kill Harry himself and he needed Harry's blood. It hurt Voldemort's pride that he had been defeated by a baby and he felt he needed to prove there wasn't really anything special about, "the boy who lived." He talks about this need in "The Death Eaters" when he says:
"You know of course, they have called this boy my downfall. . . but no
matter, I can touch him now! [Harry feels terrible pain in the next
paragraph while V touches him] I miscalculated my friends, I admit
He goes on for quite some time re-hashing stuff we really already knew from the first three books. He talks about the Sorcerer's Stone and being thwarted again by Harry and his dissappointment in the lack of a search for himself by the Deatheaters standing there, but goes on and begins to speak about Bertha jorkins and how the plan to capture adn kill Harry evolved,
"But how to get at Harry Potter? For he has been better proteced than
I think even he knows, protected in ways devised by Dumbledore . . .
as long as he is in his relation's care. Not even I can touch him
there. . . Then, of course, there was the Quidditch World Cup. . . I
thought his protection might be weaker there, away from his relations
and Dumbledore, but I was nt yet strong enough to attempt kidnap in
the midst of a horde of Ministry wizards. and then he would return to
Hogwarts . . ."
"So how could I take him? Why by using Bertha Jorkins's information
of course. Use my one faithful Death Eater, stationed at Hogwarts, to
ensure that the boy's name was entered into the Goblet of Fire. . . "
Voldemort goes on to have Wormtail release Harry so they can have a "duel" in the next chapter.
Additionally, Harry neede to be portered to Voldemort at an expected time and a particular place. There was a complicated bit of spell work and a potion involved here so just having Harry show up when ever crouch could happen to get him to touch the proper pen or whatever wouldn't have worked well.
Just before having Harry released for the Duel Voldemort says:
"You see, I think, how foolish it was to suppose that this boy could ever have been stronger than me, but I want there to be no mistake in anyone's mind. Harry Potter escaped by lucky chance and now I'm going to prove my power by killing him here and now. . ." Emphasis mine.
Sure, Voldemort could have had Pettigrew call his Deatheaters to guard the graveyard and wait and capture him and then had pettigrew start in on the potion etc. etc. but then, I don't think Voldemort trusted the other Deatheaters to let Pettigrew go through with it, he certainly didn't want all of them seeing him in his weakened state and doing it the way he did meant he could put on a show and monologue for awhile - something all Villians truly enjoy.