To be fair, just because bigs use their own frameworks doesn't mean they produce great UX. There's an added learning curve to any non-native app that users incur when this happens - things are out of place, don't work as expected, features don't exist, etc. For example:

- Facebook doesn't use standard share sheets or web views. So if you're viewing an article, you can't use reader mode and you lose the (considerable) power of share sheets to do things like sending an article to another app or take an action on it.

- Gsuite doesn't register left swipe as back even though 99% of apps on iOS do. Icons for common actions are subtly different and make you double-take while you understand what they do. Accessibility options like larger text do not work, so if you're vision impaired good luck trying to figure out what you're looking at. And there's a ton of other quirks that have existed over the years including scaling to new resolutions/ipads, problematic text selection, performance, etc. It's not as bad as it used to be, but it's still not a pleasure to use Gsuite or any material design on iOS.

While many apps may use cross-platform frameworks solely for saving developer/designer time, the bigs use them for an additional strategic purpose that is detrimental to users.

- Google wants you to be familiar with their apps regardless of platform, so that its easier to get you to switch to Android or Chromebooks.

- Facebook wants to keep you in the app as long as possible, limit social sharing outside of their app, and spy on your browsing activity, so they add friction and don't use modern iOS web views that prevent that.

I wouldn't take cues from the bigs and just assume because they do it, it's the best course of action.