Is the whole book actually worth reading? I mean the concept is very simple (although extremely important) and can be explained concisely in a couple of minutes - why a 500-pages book?

A lot of the book is going through various types of specific bias (e.g. anchoring, sunk-cost), and going over the way in which it was studied and shown to exist. I see this as valuable as because, as you say, accounting for irrationality is important, and having an indepth account of exactly how bias arises is key to internalizing the reality that we are all prone to succumbing to these errors.

The concept of almost all books can be explained in very few pages. The benefit of going through the book is it deepens your understanding, and you will be able to remember it much longer than what you will by reading summaries.

You might enjoy Undoing Project by Michael Lewis as well, it's like the backstory of Thinking Fast and Slow.