When I'm opening Uber at 1am in the cold to get a ride home, this is not the time to download a payment SDK update.
The way this should work is when you set up payment option X, it downloads the relevant payment info & then you're set from then on without any other modules unless you add a new payment option. Likely pre-bundle the generally "global" options (Apple Pay/Android Pay since those are platform-native & credit cards since those are likely small implementations).
The real reason is that you will always have drop-off because the download phase is split in two (on the other hand you'll have increase in installs because the app size is smaller). That would need App Store integration with the loadable modules so that you could say "Install these payment features of the app". This may not be a win because again, it requires the user to do more work. Simple for everyday users will often win the day even if inefficient vs more optimal options that achieve that optimality by pushing complication to the user.
I recently was traveling. I landed at a new destination and checked the internet speed. Mobile via my tablet just outside the airport was theoretically as 50KB/second via Google speed check. However actually downloading a file from US servers was 5-15KB/second because of the latency (3000ms+) being so high that packets were constantly being dropped.
That's at best, 75 seconds waiting for a download. At worst it's 16 minutes.
On the other hand, I was able to get Uber on my phone and though it was painfully slow, it found a driver in under 30 seconds.
I'd like to see them open up this possibility in a controlled way one day. Something like a review process for feature modules that could be updated in a similar process to full apps.
WRT to Apple enabling this: I imagine developers could get into a bit of a versioning hell-scape there if they were decouple updates of different modules in their app (do you know if your app is working with FeatureA v1, v2, v3, or not at all? How about FeatureB?) If Apple were to do this it would look something like app extensions do today (separate binary stored within the IPA - that's possibly thinned out and rehydrated on device); probably with very little control over what's loaded (similar to how they did rollouts: this percent on this date and nothing else)
Unfortunately the way Apple and Google set up their walled gardens makes this impossible. I guess Apple would prefer if the Uber app dropped all of that and just made everybody use ApplePay instead.