The excessive quantity of items dropped in team play leads to inflation, making trading cumbersome.

Since the first season, as more and more people formed teams to obtain more currency, it caused inflation to worsen, making solo players feel a gap due to the large difference in drops between solo and team play. Solo players lost their enjoyment of the game, and even more team players were not happy; they teamed up just for the extra currency. And if there were too many people forming teams, the currency would also lose its original value, making buying items troublesome and picking up items cumbersome. This whole series of issues, caused by teams being too dominant, accelerated the game's decline.

The party bonus doesn't work out to 100% what each player would have found had they each run the content solo. It's not that people in parties get more money per person than people not in parties. It's the opposite.

Boss-exclusive loot table is not affected by rarity, nor party bonus.
So Party play is better only for Raw Currency (rarity) or white bases (negative rarity) farming.
Temple snake is perfect suit for that, and it is pretty much the only content that's "better" for a group.
Temple can be utilized by a solo to farm Atziri too btw.

So, as a solo, you farm Boss-exclusive items instead of farming raw currency/white bases.

Groups inflating the Currency value are effectively increasing the value of items from Boss-exclusive loot tables, which is balancing the issue.
Also - more divs in the pool - more you can actually use them to roll mods on an item instead of stockpiling for a buy.