They would much rather confidently repeat a date that is totally unfounded rubbish which will have to be rolled back later, because then they can blame the engineering team for not delivering to their estimate.
In Australia, an SDE + overhead costs say $1500 / work day, so 4 engineers for a month is about $100k. The money has to be allocated from budgets and planned for etc. Dev effort affects the financial viability and competitiveness of projects.
I feel like many employees have a kind of blind spot around this? Like for most other situations, money is a thing to be thought about and carefully accounted for, BUT in the specific case where it's their own days of effort, those don't feel like money.
Also, the software itself presumably has some impact or outcome and quite often dates can matter for that. Especially if there are external commitments.
I agree there is less uncertainty in plumbing - but not none. My brother runs a plumbing company and they do lose money on jobs sometimes, even with considerable margin. Also when I've needed to get n quotes, the variation was usually considerable.
I think one big situational difference is that my brother is to some extent "on the hook" for quotes (variations / exclusions / assumptions aside) and the consequences are fairly direct.
Whereas as an employee giving an estimate to another department, hey you do your best but there are realistically zero consequences for being wrong. Like maybe there is some reputational cost? But either me or that manager is likely to be gone in a few years, and anyway, it's all the company's money...