Because they weren't good(best) enough most of the times. Let's examine the editions where Pakistani fans/media believe they should've won or atleast had chance.
87 : Yes, this one was truly dropped. The mighty Windies had lost most of their fast bowling attack & while they were still incredible they simply weren't invincible anymore. Aussies were going through a tough transition period & English weren't expected to topple the subcontinent giants. That leaves Pak & India, the true favorites. The reality is they played poorly against an average Aus. Their chasing method was as usual pretty one dimensional & unimaginative.Leaving too many runs at the end & hoping one of their top guys(Javed,Imran, Malik) will somehow finish it. This predictable strategy was not always going to work specially against a shrewd tactician like AB who knew how to get the job done with his limited resources. Pak got away with this exact approach in 92 simply because they had guys like Inzy,Moin & much improved Akram at the back end.
96 : They were hot favorites but come knockouts it had been proven they were not as good as they thought (still think) they were. They couldn't handle the Proteas & I don't think they could've handled the Aussies. Most importantly I don't believe they could've stopped the Srilankan juggernauts even with their almighty bowling attack. They underperformed certainly, but I don't see them surpassing both Sri & Aus. Simply put they didn't have the skill of the former & willpower of the latter.
99 : Without a shadow of doubt the most balanced & strongest white ball unit they ever have had. But again no way they were better than Aus & SA(the true top dog of the era). Fitness,fielding,discipline,strategy & most importantly aggression they were lacking in all of this behind Aus & SA.Inconsistent great bowling with gun lower order power hitting was never going to be good enough to win the title against such teams. Specially considering all had witnessed this exact formula in 1992. Pak needed to lift their fielding & ofcourse their so called great batters (other than Anwar) had to perform. I don't think Pak's loss was injustice. The real injustice happened with SA. Pak were good enough for semis & they actually bettered it.
03 : It's ludicrous even to suggest they had even the faintest of chances to win. But you have clueless ex-cricketers like Amir Sohel claiming they were one of the contenders!

On paper, the team looked like million bucks. But on reality they were resembling beggars. Forget Aus/SA, the reality was they were easily the worst of the 8 traditional sides. At one side you had aging legends who were still bitter about past & wouldn't admit their best days were long gone. At other end you had guys who were supposed to be at their prime(Afridi, Akhtar, Razzak, Yousuf) having no clue or plan how to uplift the team. The captain & the vice captain being the worst performers of the side with ball & bat respectively really had shown their true worth at the time.
11 : The reality is they weren't exactly at great form. They had basically lost every bilateral & tournament in previous two years. Just before the world cup they managed to defeat a poor struggling NZ side. But somehow against all odds they defeated the favorites Aus,Sri & actually managed to topple the group. That Indian side was better than them in all departments on any given day regardless of conditions. Still they had them within their grasp only to choke for the thousandth time. Apart from Wahab, Hafeez & Junior Akmal rest of them had either no clue or they thought it was still the good old 90's! That's what their body language suggested. I don't want to talk about that precious Misbah knock as there is nothing left to talk. I'll say this though, it was really unfortunate for them that their main strike bowler chose this day to lose form. Gul was breathing fire upto that point. Much like Waqar's 96 mauling, he was never the same after this. Afridi,Younus & Misbah never should have played another 50 over game after this, the fact that not only they played but went to next world cup shows you how clueless Pak cricket system is.
All in all, I would say they have achieved more or less according to their talent & vision. As for this world cup, I predict semifinal at best & don't think they can defeat any of the big 3 at knockout stages.