Because

In Alex's spatial ring, there had been two three-day Brittle Gene State Elixirs. Sylas had still never used them because he wasn't in strong need of them. He knew that Gene Elixirs were rare, and unless he could find a method of getting a steady supply of them, he didn't want to use them willfully. Plus, if he really needed one, he would just likely use his Merit Leaderboard reward Elixir.

Now, it seemed that his choice was correct as he handed Cassarae's mother and father one each.

They looked at it curiously. They didn't understand much about this world yet, so Sylas had to break it down to them. It was only then that they understood what the Rigid denomination of the Gene State meant.

With Sylas waving down Cassarae's next tirade, the middle-aged couple swallowed down the Elixir as though it was only natural. They didn't seem to have as many misgivings as their daughter about Sylas' kindness.