It seems that kpop merch is mostly fanmade... Isn't it strange?

Well, they do sell... in Korea.

If you want to buy internationally be prepared to pay more for the shipping fee than for the product.

Clearly you’ve never seen the stuff in the SM store, like bracelets and key chains tied into artist’s birthdays or rings with their signature.

There's a TON of merch for the big4 companies artists and some smaller companies too. Monsta X for example has loads and loads of merch. But I rarely see them capitalize on simple clothing as much as they should? Getting an official shirt of your favorite artists should be way easier than it currently is

I kind of prefer fanmade clothing merch online.

Is it really mostly fan-made though?

Twice in particular is known for having a ridiculous amount of official merchandise (which feeds complaints of "milking the fans"). Back in 2017, someone tied to create a comprehensive listing all of their merch, and the total cost to acquire it all was estimated at ₩31,972,500 KRW (~$28,500.00 USD). And that was nearly four years ago. At this point, the total cost could easily be three times as high I imagine.

It's called distributing the wealth