Because it is a central operator. The coins might not be there if the operator disappears with everybody's coins.

So I ended up with some onchain sats in my amethyst wallet which I check d after seeing notification. Opened wallet today thinking I'd just send them to you for devs. Amethyst says wallet unavailable 🙄

...and normie users losing their nsecs and thus unbeknownst burning coins is ok?

Yep, they would also lose their npub cashu if they lose their nsecs.