Because we can't really define what a line is. You try defining what a line is and see where you will get: nowhere. That's because a line is so basic that you can't break it down any further to describe it, so we just assume that it is what it is.
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There are typically three words in geometry that are undefined. The first is "point." A point has no dimension, length, width, or thickness. The second is "line." A line has no thickness and goes on indefinitely in both directions. The third undefined term is "plane." A plane has no thickness and has no boundaries.
a plane
It is a very basic concept which cannot be defined. Undefined terms are used to define other concepts. In Euclidean geometry, for example, point, line and plane are not defined.
A slope is 0 if it is a straight horizontal line. A slope is undefined if it is a straight vertical line.
Point is not an undefined term. It can be defined as a mathematically precise location, or as the place where two lines intersect.