"Can't you see the absurdity of the Bible?"
I understand that you are an agnostic. An agnostic isn't sure if there is a God or not, right? Perhaps you do not think that the Bible represents the real God, if he is real, because you think that the Bible is absurd.
"If it is almost impossible for the rich to enter the kingdom of heaven, then why would God create heaven as having pearly gates, streets of gold and many mansions?"
The Bible refers to streets of gold and gates made of pearl in the Book of Revelation, chapter 21, verse 21. This was part of a longer description of what is identified as "the Lamb's wife", that began in verse 9 and continued until verse 27.
The first chapter of Revelation tells us that its message is presented in "signs" or visual metaphors. This becomes clear when you read the entire book, which is only 22 chapters. For instance, the Lamb is Jesus Christ (John 1:29), and his wife is the Christian Congregation, "the congregation of the firstborn who have been enrolled in the heavens" (Hebrews 12:22,23)
The meanings of these and other signs in Revelation can only be found by referencing other scriptures.
They are not about accumulating money and possessions, because pursuing them can only distract one from the more important things, like repentance from sin, and salvation which is only possible through faith in Jesus, and changing one's life to imitate him, and gaining eternal life, which no amount of money can buy.
People who have money and are of the Christian Congregation were given this advice:
"Instruct those who are rich in the present system of things not to be arrogant, and to place their hope, not on uncertain riches, but on God, who richly provides us with all the things we enjoy. 18 Tell them to work at good, to be rich in fine works, to be generous, ready to share, 19 safely treasuring up for themselves a fine foundation for the future, so that they may get a firm hold on the real life."
1 Timothy 6:17-19
"It sounds like he wants us to be poor in life so that we will want to be rich in the afterlife, maybe? Does this make any real sense?"
He does not want us to be poor, we need to work for a living, but if this is our goal, then riches become the center of life, while faith in God and trying to please him takes second place or lower. But the verse in 1 Timothy shows that the hope of the Christian is not on uncertain riches, but to be rich in fine works, and to get a firm hold on the real life, the life that will be always happy, and which does not end.
"Update: Just think how many Bible-believing people are restrained from building wealth and earning a lot of money for their family and have a happy, rich life just bcoz of a mythical book!!"
This book would not be mythical if it makes sense. Think about it. A person can build wealth and earn a lot of money, and then die the next day. Actually this is one of Jesus' parables, in Luke chapter 12, verses 15 through 21. Please read it, and you decide whether or not his advice is mythical, or if it makes sense.