Allow Quantity to be Adjustable on Upsell Products

You can have a normal order form that allows contact to select quantities.

But, right now the only up-sell option is to sell something at a fixed price.

Why not allow up-sells to have user selectable quantities too? 

Here's the use case:

I'm selling 5 physical products on a form. Next page, I want to up-sell 5 accessories for that physical product too.

It works in the digital space too:

Say I'm selling 5 digital tickets to an event. Next page, I want to up-sell 5 VIP packages.

etc.

Surprised this isn't possible (at least from what support has told me)