Some writers/editors believe Marriage ages a character, they don't want their characters to grow up, despite it being part of life.
This is basically it. Plus there's the wish fulfillment aspect for some of the audience/creators...they want to fantasize about being able date but not feel "tied down."
Not saying every relationship should go on an on. But marvel should have a handful of characters who get together and stay together no matter what new writer comes in or what is going on at the time. They have to make marriage and relationships matter again.
And this is where I am coming from with it...not everyone of a certain age...but a decent amount that seem to make it work. whether marriage or at least long term relationships and will lead to it.