Labour costs are probably the primary barrier for this, but I can't help but wonder if a big distributor couldn't make it work; after all, the labour costs are already factored into the component unit price, not the shipping costs. For a large, traditional order you get a shipping list, determine the box size needed, gather the components, pack the box, slap a label on, and place it in the outgoing mail trolley.

For these small shipments, you would get a shipping list, see that the order system determined that it would fit in an envelope, grab an envelope, gather the components (perhaps for ten envelope-orders at a time), place in envelope, slap a label on, and place it in the outbox for envelopes.

It doesn't seem that much more labour intensive or more expensive in terms of packaging, and the first distributor in a geographical area to do so might gain a bunch of customers. Even if the base price of components was increased by 10% or 20% for envelope orders a lot of us would still use that service.

Mouser and DigiKey could offer a reduced SLA for small postal orders, meaning they wouldn't send someone out specifically to pick that order, but would do it opportunistically.