Your question perhaps should be the other way around - what is the similarity? That is because 'in a long time' is only used in limited contexts. With 'seen' and 'used' you can use either. But there are many cases where you can't use 'in a long time'. You can't say 'the tap has been dripping in a long time'. You can't say 'the house has been empty in a long time'. I think that perhaps you can only use 'in' with past participles of verbs.