What is it with employers trying to low ball you at all costs? And how to defend my value best?

So, I have been in this situation over and over again, and lately, I thought this company wouldn't pull this shit on me, but well here I am. Once and for all, I want to ask Reddit's great opinion on this topic so we all won't have to deal with this over and over again.

The business is a software consultancy, so basically we create software for money and also rent out our souls if requested.
The company sees itself to have extraordinary attributes, such as: Great work and office culture, human values such as respect and integrity and any other buzzword that comes into your mind.

I've been working with this company for a while now as a contractor and received an average salary. Nothing to complain really here. But now as I got my work authorization, they want to hire me as a full time developer. While being a contractor, they already treated me as a full time dev in many ways.

Now that I should become a full time employee, they send me an offer letter, and as you can imagine they want to low ball me.

I don't want to go into full detail, but it's a LOT less than I made as a contractor, it's a good 15k short of what my future colleagues with less experience and duties make. Sure enough, I can't take this offer as it is, especially as I feel like it's a disgrace to my skills. After all, they have seen my skills for over 6 months now and they should know that I can easily pick up any other developer job and get that amount of money. I was never really offered a discussion or renegotiation for this offer, so my question really is: How do I get myself the offer I deserve?

Naturally, I'm not great at this kind of discussion I need to have, otherwise I would be a sales man and not a dev. So, please guide me towards a solution for me problem!

Second part of my question: As a dev who thinks rationally and logically, I don't understand why this is cat and mouse game that I will have to go through is necessary. Why are companies still do this? If I would go somewhere else, I could easily get what that 20k/yr+ and my current employer would have to go through greater hassles to replace me. It doesn't make sense to me at all that an engineers value is determined by his negotiation skill. Could someone might also explain this a bit?

tl;dr - Have gotten a low ball employment offer; Need help how to properly negotiate; Also why do companies do that kind of bullshit?