I don't want to suggest that engineers/software developers knowingly mis-sell things to customers. It's sort of true and sort of not.
Organisations have all sorts of people in them with differing knowledge and attitudes and motivations. Engineers in one discipline may have faint understanding of the issues facing those in some other group but quit often they don't really get it. People who aren't actually doing the work generally have no idea - always relying on summaries from the do-ers.
In all new work (or just new to the people doing it) optimism/realism plays a role. You ask someone "can you do this" and they look at it and say "sure". If we had no optimism we'd never tackle anything new. People with experience also have caveats and qualifications but they are complicated - e.g. "we should be able do do something like this but it's not been done quite this way before so you can't be totally sure that there will be no problems." Imagine now that there are hundreds of people giving estimates and judgements that form the overall picture.
The management, who are often the people who found they were not good at technical things, are not equipped to sum all of this up into a final summary of probabilities and risk. Plus they are under pressure to assume the best rather than the worst so that they can make a favourable offer.
Even at a low level, managers are under pressure to present a good picture to their bosses and so on up the chain.