Because he has a sense of honor to him. Not because of morals, but because, as he said, he didn't feel it would be fair to "murder" -- right there acknowledging it would be murder, showing he's cold and heartless -- Link, but rather "beat [him] within an inch of [his] life". It wasn't laziness, it was just Ghirahim being Ghirahim.
And, seriously, do you really think he felt he had a reason to go all-out on Link right from the get-go? He was just some random kid with a sword that happened to have some mystical power. Ghirahim even stated if it weren't for the sword, Link wouldn't have won. Pay attention to the plot before criticizing it.
I beg to differ quite frankly.
Ghirahim actually states that he
originally intended to harm/injure, maybe even kill Link, or he thought so, with the tornado he conjured up that is. So it wasn't as if Link was this unknown figure to Ghirahim, he recognised him beforehand.
When they met face-to-face for the first time, it was an odd encounter to say the least and I think Nintendo contradicted themselves in a sense. They set out to create the overly flamboyant character, but yet he was angry, based on his tone, body language and he even states it. However, we are supposed to think that he is just playing around? Seriously go one way or the other, is he actually angry or just messing around? This is where a problem arises in my opinion, more so to do with the battle and reasoning for it.
Ghirahim states that he is going to leave us within an inch of death, he states that, it's fact right? Ghirahim is an overly powerful demon, who is on a another level to Link, who is yet to wield any true power and he has the most weakened form of the Master Sword to be. Now the battle was quite casual, fitting in with his halfish, not-bothered persona, but yet this just makes him seem incredibly weak, so what if he's messing around? Fact is that that he came off as weak, there is no denying that -- an antagonist should
never come off as "weak".
Also getting back onto Ghirahim's statement, did he beat us to a near inch of death? Well unless you really struggled with the battle itself, then no he did not. Like I said it was extremely casual, yet he was angry beforehand, contradictory again. Fact is that the battle was not that hard, the intensity was not enough and his persona just didn't make sense at that time, he made a bold statement -- beating someone to a near inch of death is a horrid action -- some may say worse that actually murdering someone, as he's literally going to let him suffer. Well Ghirahim failed, we beat him and he even lowered himself, somewhat, before us, we were victorious.
So Ghirahim, not only did he contradict himself, but he failed quite miserable in his actions and Link, at his most vulnerable and possibly his most weakest, defeated one of the biggest threats in the game, quite a bad move in my opinion.