why do we feel, like/comfortable speaking to new people rather than to known ones?

Really?

Or is it you like that, and assume others do too.

What is your evidence?

I also found natureboi's claim to not reflect my personal experience. I feel comfortable talking to old friends, and also new acquaintances. I guess I'm an extrovert. Never felt shy about talking to strangers.
I have traveled solo many times on over 100 trips, and have made friends on some of them that I have kept in touch with throughout the years. That was part of the benefit of travel; to meet people around the world. I have friends in Russia, France, England, Cuba, Mexico, Singapore, and all across the US.

Keep in mind CI, that being shy and being an introvert is not necessarily the same thing.

I'm in no way shy, and can flare up a conversation with complete strangers (one on one or in very small groups), or stand in front of a large group of people and talk and not be nervous. However, I consider myself very much an introvert. That's because when I do those things I still feel very much "in myself" if that makes any sense.

I avoid situations where I don't feel it was my choice to be there. Then I feel I have to be "outside of myself".

I also talk openly, freely and sincerely with people I consider friends and have known for years, even decades.

I understand many actors are the same.
My experience was different from many other Japanese Americans whose reputation as I was much younger was of the quiet American. I have made presentations to large groups as part of my job, and felt comfortable.