One factor is the time of day. When I meet people I'm usually busy with work, my thoughts or trying to achieve something else than a good discussion. When I'm online I'm alone, often in the evening, I have plenty of time and space in my head to get interested in arguments, viewpoints, and the experiences of random people I don't know.
I can also use my time effectively online: thanks to the asynchronous element of internet discussions I can find topics and arguments as I please and I can also withdraw from a discussion if I see the thread is going in a direction that won't be interesting. When talking with someone it's much harder. Thus, IRL, good discussions tend to happen with good friends who I can trust to not waste my time, and during specifically allotted time slots when I'm meeting them, and we both also have to be in the mood for a serious conversation.
I can also use my time effectively online: thanks to the asynchronous element of internet discussions I can find topics and arguments as I please and I can also withdraw from a discussion if I see the thread is going in a direction that won't be interesting. When talking with someone it's much harder. Thus, IRL, good discussions tend to happen with good friends who I can trust to not waste my time, and during specifically allotted time slots when I'm meeting them, and we both also have to be in the mood for a serious conversation.