It's a combination of margin preservation and product positioning in each market.
Common sense would tell you if it's just a matter of return availability, we'd see units a few months immediately after new product launch. M4 iPad Pro and iPhone 16 would be in the refurb store a long time ago. Obviously, it's not a logistics constraint.
So what's the reason?
Apple positions the iPad Pro as a premium, secondary device. A 13-inch iPad Pro costs 60% more than 13-inch MacBook Air. This means buyers are price sensitive. They don't need to buy one. Refurbs just push down the price of new iPad Pros.
MacBook market is far more mature. Refurb Macs help convert Windows buyers rather than cannibalizing new MacBook sales. That's why Apple sells refurb Macs so quickly.
iPhone commands maximum price discipline. Sales are very price inelastic. People are very willing to pay to have the latest iPhone and that's why Apple only sells iPhone 15 as a refurb.