I think a lot of people have already made some very good points here, but I'd like to try to make one more. The problems of the late 19th century were not problems with Laissez-faire. They were problems with a developing nation that was technologically and educationally primitive relative to where we are now. A 21st century libertarian nation wouldn't look exactly like a 19th century one, because free economies grow and develop new innovative solutions to their problems over time.
On the other hand, socialist economies tend to stagnate. Can you imagine where our country would be now if we had abandoned laissez-faire at the start of the 20th century the way Argentina did? Americans were poorer than Argentinans in 1900, and we still would be.
Why do a lot of the libertarians I talk to want a laissez-faire system? Short answer, they see many regulations hinder innovation and economic progress. The main reason that Libertians prefer the system is that it freely allows people to start a company with very little red tape or waiting. They also see regulations as ineffective or just slow down the process of economic progress. What did the libertarians response when you asked them this question, if you asked? Are they fully aware of how bad that would be? Depending on which Libertarians and how much of laissez-faire approach they want. Different Libertarians have different views on economic policies. So which one do you think would make things bad? Why policies would make things 'bad'? What standards would need to be violated to be considered bad?
Have they not forgotten about the terrible conditions that prompted The Jungle by Upton Sinclair? That would imply that you couldn't or didn't ask the libertarians you talked to in your first question. What libertarian polices would cultivate an environment such as The Jungle? Do all Libertarians think like this and agree on that type of method that would make a Jungle-like economy?
"Why do a lot of the libertarians I talk to want a lazze-faire system?"
I think all libertarians would want a lazze-faire system.
"Are they fully aware of how bad that would be?"
That question assumes that it would be a bad system regardless of the current culture and economic climate of the area where it was implemented.
"Have they not forgotten about the terrible conditions that prompted The Jungle by Upton Sinclair?"
Most of the have read or heard of that book. I think the situation is more complicated than you're giving it credit. Governments do not necessarily prevent situations like the Jungle from happening due to corruption and other issues. Simply stating "let's regulate it" is a non-answer because poorly implemented regulations can cause other problems. Also, you're assuming that a Lazze-Faire system does not have built-in mechanisms to prevent situations like this from happening.
Yes, I think property is theft if not all people have the same starting position in life. Now I don't have a better solution, but that doesn't mean I can't have the idealistic view that all men, and women, should be born equal.
Alright, you will kindly give up your home, your car, all of your food, all of your clothes, every piece of property you have, including the shirt off your back. You might have spent forty hours a week to make the money to purchase it all, but tough shit; you're stealing it from those who didn't have the same start in life as you (and even you should know there will always be such people).
Nope, I don't see a contradiction there. Because apparently it doesn't matter that my t-shirt and my food were made by people who work 60 hours a week and still can hardly afford to feed themselves. Because that's fair, isn't it? Why should they not be allowed to have the luxuries we have? So that we get what we think we're entitled to after a whopping 40 hours of extremely brutal work. Probably in the office sitting our ass off all day. Poor us.
Do you own a product by Acer, Apple, Dell, HP, Motorola, Nokia or Toshiba? Do you own a PS3? If yes google foxconn controversy. Yes, I know this has changed since then, but that's because it became high-profile, and many other companies have worse conditions. Did you buy fireworks? The people making them get 10$ a day. (Source: 'Der Spiegel January 2013)