I wonder if it's just one of those times when you look at something for so long that it doesn't make any sense.
A dream come true
a kissing girl
They both have words missing, "A dream which has come true" - "a girl who is kissing" - I'm not very good at the mechanics of English, yet, but I see from the 2 examples above that we can compare the parts
true and girl are not the same things -
a dream and kissing are not the same things -
This is not a good answer but sometimes English is better to just accept than question, I wonder if we could imagine a dream, sitting on top of a pedestal - people could look and say, it's a dream which has not come true - or they could look and say, "it's a dream come true".
Personally, I would say both saying, I would say, "it's a dream come true" as a thing / event - and I would say, "It's a dream which has come true" - if I was trying to inform someone of my desire for the dream/ desire to become a reality, and to inform them that indeed the dream did become a reality.
Perhaps it means the same but it's just a shorter way to say it, like we say "I'm on the bus" instead of "I'm sitting on a seat which is in the bus" - we like to make things shorter and easier.
I'd very much like to hear a professional's opinion on your question.
Good question, thank you.