That’s unfortunate. You could probably press them to see why. It’s a completely valid way of shipping things, so I’d be curious why they won’t do it.

I used general delivery while hiking for 5 months and had gear shipped ahead to the next town I’d be in. It’s possible the companies that I dealt with knew why someone like me needed general delivery and I got lucky.

I’ve used it with success, but you have to be specific with the addressing of your package (mainly just the “General Delivery” in the second address line). Details here.