Because plastic producing companies have spent the last 40 years and a lot of money telling us that plastics are good "because they can be recycled" while not recycling and not addressing the very real problems caused by plastics because profit.
I don't know about "has become". I've known it to be the goal as far back as the 1980s. (Anything before then is before I started forming memories.)
I think it probably comes from the same place as organic food and suchlike: a conflation of "more natural" with "better for the environment." They're not necessarily the same thing, and we're finding increasingly many situations where doing things in a "more natural" way is actually pretty terrible for the earth's ecosystems when seven billion humans are trying to do it at a planetary scale.
But why?
Waste plastic belongs in landfill.
It’s crazy to try to address the plastic waste problem by keeping it in the environment, as though “it’s useful so it’s OK” makes sense.
Just dispose of it safely.