Everyone has their own reasons.
Many, read most, aren't going to sell seeds and send them even within the same legal State. That's a federal deal and they already have enough at risk without that. Selling them to walk in customers is no more risk than they are already subject to.
I don't see US seedbanks becoming a big thing until Federal Law changes here. In general I find US banks have a smaller selection. Personally considering current laws and the agencies involved I feel it's more dangerous for growers to order from US banks than from another country. Customs doesn't and isn't going to chase anyone down for a few seeds, esp. when they can't get at the supplier. Now with the USPS and an Attorney General that hates cannabis and would love a way to get into States that are currently under some protection, I can almost guarantee that at some point they will be targeting shipments from legal states, it'll just take some time for them to find a pattern to target. But that just my opinion.