Because later on Decent Challenge mosnters dont seem to be decent anymore, and they get harder and harder, soon you wont be able to take em on before dying. And if you do win, you wont get as much exp as a normal exp party. You can kill Decent for 50 exp and it would be lets say 7000 til the next lvl...and thats alot of Decents and near deaths since thats hard as you lvl up beyond lvl 10. In a party you can fight Very and Incredibly Tough enemies and if all goes right you can have up to 175-200 on non-chains(chain=you fight another monster within a certain ammount of time and beat it in a certain ammount right after another, has to be Tough or higher lvl to be chainable) and then up to 210-300exp on all chains.

So its like:

Solo: 50 exp....rest(long time)....repeat. If you die it takes longer to get exp back. Plus if i dare solo any Decent Challenge at my lvl 50, i would be creamed.

Party: (assuming same mob and toughness all the way) 175exp...finish fight...45 secs later...200 exp...then 232 exp(chain #2)...250exp...then 275 exp...then if you got an exceptional party...300 exp (chain #5). Rest a little bit...then repeat.

Everything is just faster that way. This game is BUILT for partying...no way around it unless BST..and its slow too.