I have a new iMac Pro, which I bought from Amazon, and I'm plenty unhappy with not being able to buy a Trackpad 2 to match, and I'll tell you why this is a terrible policy.
I'm a retired IT Manager, and I'm thinking of coming out of retirement to start up one more company. These computers are expensive, complicated tools intended for specific, serious jobs, with lifecycles of years to come. Apple's apparent level of service for this, their premium, halo workstation, is 'No.' You (the client) can't have reasonable after-purchase support. No real reason -- just No.
What if a Trackpad is ordered, from Apple, at time of purchase, and it is lost, or stolen, or broken. Or a designer/programmer/editor decides they need one after the order. Or one employee leaves, and their replacement can't use a mouse, and needs a trackpad.
The point is -- these are not sprinkles on a cupcake; these are expensive, serious, complicated tools. If Apple is not going to provide even minimal after-market support, they should say so in advance. This is a terrible policy from the perspective of IT departments that need to support working professionals. Dell never told me 'no, you can't order that. You can only get that if you ordered at the time you ordered the workstation.'
And I have tried, and been told no by four different Apple people. I would have, and should have, bought a high-level regular iMac, if I'd known about this idiotic policy.