I don't understand why managers are not held responsible for the under estimated timeline of the project, the developer will suffer from the overtime and crunch of coding.

Who should do that?
As long as the customer is happy (enough), and the company is making money, the higher management won't complain.

If you have a union or other workers' reprentatives: I would guess it is their jobs to hold them responsible, so complain to them.

@sbiewald that's probably a reason why big companies hate unions

@DevRage The whole purpose of unions is to push back against companies. A union that doesn't help its members to bargain for better working conditions is a fraud and a disgrace to the name.