The rewards for the challenges don't match the effort and time required.
40 revives is what half a level? Are you serious? That's absolutely insane. 40 revives takes A LOT of time and effort even as a weekly.
If you're keeping these weekly challenges this long, every single weekly needs to reward a level
This is one of the things we're fixing, per the tweet linked above.
Should be much more achievable after the change. May not become apparent until next week's challenges roll out (although devs are trying to figure out right now whether we could get it out sooner).
One other problem I'm noticing is that "weekly challenges" are actually sort of incorrectly named... because once they unlock you have all season to complete them. In marketing jargon, it's not great positioning.
Regardless, I get where you're coming from.