A Rogue player that wants to gain Sneak Attack damage as often as possible can already get close to "always". All she needs to do is attack an enemy which is already in melee with an ally. This is in general easier than having advantage on attacks, even tho the latter is presented as the main way to get Sneak Attack, however there are situations when this is not possible: e.g. when everyone is attacking at range or with reach weapons, or when all the melee allies are incapacitated.
So would it break the game to just default to "always"? Probably not. But it will fundamentally change how the Rogue is played tactically. Because as it stands, the Rogue has to look for ways either to have advantage on the attack rolls (hiding, gaining higher ground, etc.) or to target specific foes which are in melee with allies.
It's maybe a matter of preferences, but I definitely like being forced to think how can I get that bonus each round rather than not.