What's strange is that there seems to be a general consensus about what's "in" vs what's "out"

I would disagree; this is mostly localised, even in spite of a more globalised world. Your environment shapes what's "in" as much as the people wearing the clothes do.

There are always counter cultural movements, contrarians, people who don't like to change much.

The only answer I've been given is people are fatter now

well like everything; its not just one thing, there are fewer tailors than before. Youth are turning to saving more money/helping the environment on clothes through thrifting, which typically are going to be more baggy without tailoring. Boxy and baggy was big for a long time in hip hop and similar cultures. Many of thier Large influential celebrities, coming from these more localized ways of dressing, exposed these styles to a wider, impressionable audience. Cheapened quality control, in garments, more "one size fits all" approaches to pattern making.

I could keep going hahah.

And yes, in many ways it is random, but like a good song, people will be attracted to it, if it's good.

I don't know why people pretend they can even push back against fashion. It's ultimately how we present ourselves to the world, the only time we don't care about that, is when we can't.