In Native they use default cursor on a button, while in the web they use the hand cursor on the buttons
In both cases they are following the standard interface convention for the platform: desktop apps typically do not modify the cursor for clickable elements. Web apps typically do. (Touchscreen apps are irrelevant to this question, because there is no cursor to modify.)
There is no good reason to break the web UI convention of "pointer cursor indicates clickability". It's standard, it's effortless (you would have to do extra coding to prevent it happening, in fact), and it does serve as a useful -- if minor -- visible cue to the user. (It may often be a redundant indicator, for elements whose design already indicates clickability -- but not intrusively so; there's no particular drawback to that redundancy, as it adds no screen clutter or cognitive load.)
For desktop apps, you would be justified in following the standard convention of not modifying the cursor for clickable elements. (When in doubt, "follow the standard convention" is usually the right thing to do.) Or you could equally defensibly follow the web standard and use pointer cursors, on the grounds that it's nonintrusive, harmless, and potentially useful.