Because many BAD THINGS happen if you have bad fuel and NONE of those things are things you want to find out when in the air.
The small planes I flew all had small valves on the bottoms of the wings to check the fuel. You would get a small sample, check it’s color and then toss it out on the ground
AVGAS 100LL (which is super normal for piston engine planes) is light blue. If the sample isn’t that color, DO NOT FLY THAT AIRCRAFT.
I am not a fuel expert, I just know what it’s supposed to look like. If the fuel looks good, it’s an indication that the aircraft is safe to fly.
Remember,
“There are old pilots and bold pilots, but no old, bold pilots”
—E. Hamilton Lee
So, use caution, follow the check list. You’ll live longer.