H&M is much, much bigger than Forever 21. Forever 21 barely exists out side of North America, but H&M can be found everywhere. I mean everywhere. That may be a factor in how cheap they can be - The scale of the business.
I’ve bought a $30 dresser at Ikea and $7 T-shirts at H&M, and both were about as durable as you might imagine. I’ve also bought a $200 dresser at Ikea and a $100 coat at H&M, and they’ve held up as well as comparable products twice their price.