Riot is in firm belief based on market analysis that people who have friends that play the game will keep playing it longer, and therefore have a higher likelihood of buying stuff since they play it a lot, because their friends play it, and their friends buy stuff cause they play it alot, cause their friends play it.
So theyre trying to force you to find 4 other people to be "friends" with so you play together, cause you want stuff.
Theyre worried about player retention like lots of game companies are, and they trying to CRAM HAVING FRIENDS THAT PLAY THE GAME IN YOUR FACE based on your desire to get stuff.... you know... by alienating solo players. Let me put it this way. If there was a club called "League of Legends" and in that club were 8 people who didnt know eachother, and 5 friends were waiting outside... Riot would push those 8 people out and let the 5 friends in. Riot feels that the 5 friends will stay longer and buy more drinks, so fuck those 8 strangers.
Solo players have been second class citizens of League for a while now, and this is just one more bullet point by Rito letting us know that we dont count because we dont fit their "long term player retention scheme" and we can just fuck ourselves out of missions and other stuff because were playing solo.
Sounds like Im being mean to poor ol Rito? Im not, you people are numbers to the corporate lords and masters, and if this game isnt making money then someone will be fired. Its how a business works. They do not care if you think something isnt fair if they think they can get money by doing something a minority of payers dont like, they will do the thing that makes money.
So to the OPs point again: Why? $$$$$$$$ they see represented by the extension in play time over years that a group of friends playing a game, thats why.