You won't even think about trading under pressure if you are learning to trade the right way. You cannot trade under pressure if you read trading books, have mentors, be a part of peer group of traders. The people who are likely to trade under pressure are those who are greedy and want to make it a get rich schemes. Trading for them is short term and not a long term endeavor.
I use to wonder why people always put themselves under pressure of any sort just because they want to get rich faster, moreover everything is a gradual process and so it needs time to pass through every stages but just like you said, greed has taken over them, it's just like gambling that many others want to get quick money from all because of selfish gains other than gambling with what they can afford to lose. And mostly they fail to understand what works for the next person may not actually work for them in most cases.
Most people who go into trading under pressure never had the opportunity to pass through mentorship because they can't pass through series of time only to rush into trading when they know they can't get anything good trading under pressure, aside the greed you clearly mentioned I think most of the traders who fall under this category are basically those who sees trading as a thing of luck just like gambling they hardly believe that knowledge has a good role in trading for anyone who wants to be successful not just in trading but most of the things that involves investing money. I don't know if there are people who started trading and making profit without following the learning process I think those who actually claim to be making money every now and then on the Internet could be the major motivators of those who trade under pressure because of the things they upload without proof.