Look at it this way. In a two player game (how I usually play), we both take a quest and then do both quests and return to the trader. Doing it this way, we need to do 20 quests per tier (10 each). Yes, doing two at once instead of one at a time is faster than doing two quests one at a time. But it is still slower than doing one quest solo. This means that you might complete two quests faster with two people but you won't get both player their quest point as quickly as solo and those two quests only count as one quest for each person and takes longer than doing only one quest solo.
Let's toss some numbers in here. Note that these are just flat numbers to visualize things and aren't intended to be accurate (everyone's numbers will vary anyhow).
Solo time to complete one quest: 10 minutes
2 player group time to complete 2 quests (one each): 15 minutes including travel time between the two quests
Solo time per tier: 100 minutes
2 player time per tier: 150 minutes
Solo time to tier 6: 500 minutes
2 player time to tier 6: 750 minutes
That is more than 4 hours longer real time. And that assumes you could maintain the same speed to complete high tier quests, which you can't. I could put in examples that scale time by tier, but I think you get the idea. But let's say your average times to complete quests from tier 1 until tier 6 is:
Solo: 20 minutes
2 player group: 30 minutes
That is over 8 hours longer real time. Again, these are example numbers. Some people will take longer and others will be faster. Any changes will affect the difference in time. But even with the fastest players, they are going to take longer than the same players doing quests solo.
And this is just for 2 players. This divergence in time expands with each additional player.
Yes, you get more rewards by doing more quests, but the rewards aren't worth much anymore, so not really a big deal.