Because every single time you don't, the end result after the prototype presentation is "oh, you're 90% of the way there, it'll only take 10% of the time you've already spent to finish it!".
I'd say that leverage in negotiations sort of comes into play, but consider that I may have hired a person for 100K and I was told that I can hire someone else, but I only get 80K this time because that's all that can be justified with the number of accounts we have. So, I hire someone at 80K.
If Ms. 100K and Mr. 80K start talking, there are all sorts of possible problems, but in the end, though, I was only given 80K to hire someone. If you didn't accept that, I can't hire you. Would you prefer to have not gotten the job? That will depend on if you were in demand, I suppose, but I'd usually say that if 80K was acceptable to you, then you're not losing out.
That's why you probably shouldn't talk to other people. You might well be convinced you should be making 100K, but if you'd insisted on that number, I couldn't hire you, so you'd probably not have a job. Also, Ms. 100K may have been hired while the company was doing very well and was able to be generous. Instead of dropping her salary or laying her off in a slump, we kept her on. We can't afford 100K people anymore, but we want to be fair to her and maintain our word when it comes to what she makes. Should we have instead laid her off or knocked 20K off her salary so that you could feel better about yours?
You need to find a number that works for you, and you need to insist on it. If you get it, you should be able to do everything you wanted to do with that salary. Don't worry what other people make, someone is always going to make more than you. Understand what you are happy with and get that. If you need to adjust, then it should come from your own needs and not a comparison between you and someone else except in the most basic of fashion (such as salary research for your job description) to get a basis for what is reasonable.
At a previous workplace the opposite happened. They tried to hire an inexperienced new guy for a secret, higher salary than the experienced veterans were making, because they needed more capacity quickly. The experienced people found out, it destroyed their trust in the company, and most of them left within days. They had been working knowingly under their value because they knew and trusted the company, and liked the work. Squandering trust can be very expensive.
And that's what irks me most about your post. It's a bit arrogant for the salary-setting entity to believe they are smarter than the people working for them, that they know best, and that it is in everyone elses best interest not to know what's going on. The implication is not to trust people.
Me, I'd rather work with people I trust. I've worked at a really good place that was very open about money and why we weren't making that much of it. I still do contract work for these guys and I know exactly why they pay me what they do. They have a very good set of very skilled developers who are all knowingly making less than they might make elsewhere, but simply happy where they are. The access to all this information doesn't make them unhappy. It makes them smart, informed people making the correct work/life balance decisions for themselves. I love working with them.
In your particular example I'd much rather you'd come out and say "normally we'd pay 100.000 but we're very low on budget so we have only 80.000", to which I could then reply "I can accept 80k for a 1 year trial period by which time we can both assume either your strategy and my work will have brought the company profits up so that you can start paying me 100k, or I am free to pursue other options". I'm sure an arrangment like that open and above board makes everyone happier.
It is now pitch dark. If you proceed, you will likely fall into a pit.