You run the business like that for years with no issue and everyone is happy. I see the business model and think "I can do that". You and I competitively bid similar projects.
Instead of billing the client 1000, you end up bidding the job for 600. It takes more time than you thought and you pay your folks 650. You lose 50 on the job.
You tell your shareholders don't worry I have a really big job a 10,000 unit job. I bid on this job too, you win with a bid of 6000, it takes more time than planned costs 7000. You need to borrow money, but its OK because you have several 10,000 unit jobs on the books. You lose on these jobs and need to borrow more money. Either you start to make a profit and work your way out or you go bankrupt.