Why people is so impatient?

And some people were complaining that the mew quest was "too short"

I for one don't mind the extra padding. At worst it's forced, but as mentioned Celebi isn't a must-have attacker so there's reason to rush beyond being the first among your friends to get it.

I don't mind it at all, in fact I wanted it to take a while.

What I do like though is that the challenge seems to be bit more consistent, so that all players should be able manage it within a couple of weeks. Progress in the Mew quest could vary wildly depending on the circumstances of the player, the evolve a Magikarp requirement being a good example; those in the wrong biome could find it a real struggle, while it was a cinch for others.

People want to get Celebi in 1 day so that they can go back to complaining about how bored they are with the game.

I think its the forced padding from the X days catch and the evolve eevee ones. I dont mind them making it take longer. Personally, I dont like they make you need to set your buddy to an eevee twice when I could be walking my dragonite, the first catch of the days one I dont really mind too much, lets me space things out when i work on the research

Because there is nothing else to do.
You can go grind your 250eth Beldum candy or go catch a brown trashcan that you don't care about at all...
You can save up more rare candy or something.
Either you have all the important candy and mons or there's nothing changing about the situation in a short period of time.

So you can rush to get Celebi which is new.

Because they're trying to get into the GWBR for "Fastest time to get Celebi." Now that I say that, it sounds even stupider than before I said anything. xD

It's to keep people playing. People that play everyday shows Niantics Investors that their money is safe with them.

It's an obvious time padding that doesn't teach new players anything or add a challenge in a game woefully short of challenges... when the only challenge is beating the system that's what players are going to do....

Because they are curious about the later steps in the quest and to try out Celebi.

Because the difficulties of the tasks are not evened out. Waiting for days to hunt a ditto or ghost type pokemon is ok, but to just catch a regular pokemon and spin a pokestop is ridiculous.