The idea of a scrim is to trial run pre-determined ideas against a real life opponent. It's not a friendly match where it's a first-to-13 trial-by-fire, but rather both teams saying "this is what we want to practice" and then going through with what they're setting out to achieve.
Adapting to your opponent in a scrim can actually be pretty unhelpful, and I'll give you an simplified example to illustrate this.
If, for instance, team #1 wanted to practice their A hits, they would hit A 12 times in a row. They would have also communicated to team #2 that their plan was to hit A 12 times in a row. With this information, team #2 could theoretically stack A, or brawl for A Main, or push out Mid and B and retake from multiple lanes. But that wouldn't benefit either team; it would be a waste of team #1's time, and team #2 would lose a scrim partner. Instead, team #2 would play as if it's a normal match. The point is to simulate a realistic situation so that the teams can figure out what's working and what's not, and move forward with a better understanding of what they need to accomplish before their next match.