I started to work in a big company 5 months ago. My probation period is ending this months and after that my notice period is 6 months. I already found a new job and my plan is to quit next week. Here is the problem. The job conditions are quit nice. 37 hours week, 30 days of holiday, no stress, every 2nd Friday a hack day, interesting projects and some other small things. The problem is that I still don't feel like staying there. The colleagues are nice and competent, but you just can't talk to them. During the lunch almost no one speaks. I come in the morning, say hi, have a standup and say bye. Then the office is also located outside the city where there is nothing going on, so no one leaves the building and I feel like in a jail. All of this seems silly, but it makes me unhappy. I need that social part at work. I knew that this won't be a startup when I signed the contract, but I underestimated the importance of it. For me it is clear that I have to go. I just don't know what to tell my colleagues or my boss. I can't say I'm not happy with the social part of the job.
First, you don't really have to tell them anything. Just say you've found a more suitable position and you can't justify passing up the opportunity it offers.
Second, you probably should not explain your real reason for leaving. The way your colleagues currently behave is working well for them. If you reveal that you can't tolerate that behavior you may cause them a mild emotional distress but will not help them in any way.
It is much better to let your former colleagues guess about your true reasons for departing. If they are interested in the why, they will come up with possible reasons and will be unable to decide which is correct. But whatever their theories are, they will never guess that you just don't like working with them. Let that be your little secret.
Why can't you say you're not happy with the social part of the job?
I don't really see a problem with this - you want something culturally different. If you want, you can say there was a cultural fit problem, and you wanted a more social feeling.
You'll probably find that what you want is generally lacking in most companies. Tech ppl by and large are relatively anti social.
I would suggest - simply because I felt the same when I started and I wish I had done this - that you move into sales. It is much more social, you will earn much more money, and the lifestyle is probably more to your suiting.
But saying cultural fit - wanting to talk to coworkers - is not a deal breaker, and given it seems your company is trying to make a better environment something management will want to know.