Why it's impossible to mimic an IP address?

Your IP address can be any 32 bit number you choose if you don't have DHCP choose it for you. But, the routers will only forward and respond to addresses that have been correctly configured. There are conventions, standards and protocols that define what addresses are valid and how they should be used. In a lot of cases, invalid IP addresses simply get dropped.

If you'd like to learn more, this Wikipedia entry is a good starting point: https://en.wikipedia.org/wiki/Internet_protocol_suite

Nothing. Barring some routing-level tampering, you simply won't be able to get a reply if the IP you put in your packet is wrong.

And for pretty much everything you want to do as an end user, you really want a reply.