Premade should not be this much of an advantage.

If anything it should probably put you more at an uphill battle to compensate for the potential coordination that your premade could bring to a fight. But equal ground is also perfectly fine. Since it is outside of RIOT's control as to if you can group with someone or not, it should not be too punishing if you can't nor be too rewarding (at least not as much as it is now) if you can.

Premades often use voice chat softwares that are not provided by league. While many are free to use, there is a considerable (almost always) lack of use by non premade members to communicate with each other since many have different software (skype, curse, teamspeak, etc...) or no one wants, or thinks to start a call. It also is not very fair for people who can't use voice chat software, such as if they dont have anything to use as a microphone, or if they can't run the two clients simultaneously without noticeably impacting the performance of the game.

This might be more of an issue the further down the rankings you go, since I can imagine that most high elo players have at least a duo they play with and have the proper equipment for voice chat to work fine. So this may be minimal in how it impacts competitive play. But lower in tiers it is a major, and sometimes the deciding, factor to a game's conclusion.

The hard part is how to "fix" this.