Why does an admissions committee need to look at the whole transcript? Admissions committees look at a student's entire transcript to get an understanding of their academic abilities. Why is it not sufficient to judge a student's academics based on GPA alone?
The problem with GPA as a measure is that it just mixes together a bunch of different classes indiscriminately.  Consider these three different students:


This student is pretty steady, but not great, and tends to get an even mix of As and Bs across all of their classes, obtaining a 3.5 GPA.
This student always takes the class with the easiest grading policy whenever they can, and tends to get mostly As in the "easy" classes. When they cannot evade a "hard" class, they get mostly Cs, resulting in a 3.5 GPA.
This student is very strong and gets mostly As, but there was one semester when family problems disrupted their life, and missed assignments caused them to get Bs and two Fs, also ending up with a 3.5 GPA.


As you can see, even though all of these students get exactly the same GPA, they are very different. Student #2 is clearly far weaker than Student #1, who is in turn weaker than Student #3, whose GPA has been strongly affected by the one  problem area.

If we can see which classes they took and what grades they got on those classes, we still don't have the full story, but can get much closer to an appropriate evaluation of each student's actual strengths.

This set of pages at Berkeley may be useful.