Because it's not! Glad to help you clear that up.

Same reason you might use 22/7 instead of the exact value of π. If I look at a clock and see it's about ten to 2, it's rare to never that I actually need to know it's 1:53:22.57365785285978520256734567314854372354675466099.

They are actually a helpful way to show passage of time visually, without abstract math knowledge. For example my son has downsydrome, he could read time from analog and understand passage of time and time left on it, but numbers counting up to 60 was abstract.. Like its 47 minutes past 5 how close to the hour is it getting? No clue unless he wrote it out as a math question and did the subtraction. But for him those were meaningless numbers anyway. 15 was no different than 45 for him. But visual cues of quarter past and quarter to made sense for him