A reminder that Hypnotic Pattern does not actually stick around for a full round. The spell clearly states that it appears for a moment then vanishes, much as a fireball's effect does (flames spread out quickly then next person in initiative goes).
It's a brief flash so the caster can close eyes then open eyes before another creature can take advantage, assuming that another creature had not been waiting with a readied action to attack during the brief flash of light.
The concentration duration is for maintaining the hold on affected creatures after the flash of swirling lights.
Nobody is claiming that the flash lasts for more than an instant.
However, all combat is kind of happening simultaneously, we just break it up into turns for convenience. In addition, from a metagame standpoint it makes sense that there should be a penalty for excluding someone (including yourself) from a spell. After al being able to sculpt spells to exclude individuals is one of the big reasons to play an Evoker.