Why do some people have this urge to impress everyone asap when they join a new organization?

To hide their shortcoming?

But first impression is the best one some say

Why is that a bad thing? It's very natural to want to fit in, do your best work, and become quickly known as a great developer amongst your peers. It's what I'd advise to anyone starting a new workplace - hit the ground running and get yourself a great reputation. First impressions count.

To build trust.

Because they want to help
Because they fell under pressure to prove themselves
Because they’re worried that they will get cut (if they don’t prove themselves) during their probation period and be jobless