The idea of buying clothes then donate them after 6-12 months has always been broken. With growing kids (or if you gain/ lose a bunch of weight) you don't have a ton of choice. For most adults, the only way you can really ensure you have an environmentally friendly wardrobe is by purchasing carefully and keeping clothes until they genuinely wear out. For jeans and good quality jackets, that can be decades.
When you donate clothes to St Vinny's (Goodwill/ Salvation Army/ whatever), they keep the best/ resealable clothes and dispose of the rest. A bunch gets dumpstered. The only reason they accept used clothing is because of the small percentage they can actually resell.
[1] Excluding underwear and socks!