That said, I never got an offer from something like SV startup or a FAANG, so I can imagine it's totally different over there.

It's not.

They can choose to be dicks about it or not, but the way they use their leverage isn't any worse. Competition for employees is fiercer than other markets. You yourself can get away with being a dick to them.

Probably more common is an experience I was in- we found a good candidate, he wanted to finish other interviews before making a decision and I gave him a few weeks to decide, then later I regretted giving him so much time. It meant I had to either pause the candidate search for a few weeks or potentially waste my time and candidates' time interviewing people when there may not be an opening.

Of course if you personally were on the job market and were actually desperate because your spouse said "maybe its time to be realistic" and the bills were piling up, then you don't have the luxury of playing this game.

That seems unreasonable in a lot of situations. In my current position, yes, they did want me to start right away but I really did want to take a real vacation before I started so I negotiated it out to a month or something like that. No, it's not reasonable to offer to start in six months. It's pretty reasonable IMO to offer to start in a month. (I did accept the same day. No brainer. It was a good offer. It was my first choice company. I really needed to move on.)

This depends very much on the industry and the location. In much of Europe, particularly at senior levels, 3 months notice is the norm and between one thing and another you might start at the new company 4-5 months after accepting the offer.