To be fair, the materials and labor used by high end brands (at least the ones you mentioned) are different from cheap brands. Most of the labor is Italian or Portuguese, and those workers get a paid month off every year in addition to benefits, even if they aren't highly paid. Likewise, the materials they use are generally higher quality and are sourced more carefully. High end brands also do a shitload of QC.

I'm not a fan of $100 t-shirts but there is absolutely a difference including much fairer practices in how the garment is manufactured.