This has been discussed quite a bit, and the devs are aware of both sides. Multiplayer challenges do take longer, and those who go for the rewards should benefit in some way over those who refuse/never try.
I play mostly single-player, so truly I appreciate your point, but the multiplayer aspects add a challenge. The single-player challenges and tasks are incredibly easy; I've completed all masteries for this season in maybe three sessions. 61 days and 12 hours till there are new ones. All daily tasks take less than an hour. Same with the weekly tasks. For OTLs (with randoms), it's the same story, and you have a week.
The players who wish for harder challenges see it as a benefit, and those who want only single-player see it as a travesty. Without taking away from either side, where should that balance point be?