Yes, it has historic roots. The term "linear" is much older than "affine" and the function $ax+b$ is "linear" because its graph is a straight line. With the invention of linear algebra, the meaning of the world linear changed. But educators, especially on the lower level are very conservative, and also reluctant to introduce extra Greek terms (which on their opinion intimidate students). So the terminology in lower levels of mathematical education lags behind the development of mathematics.
Similar situation we have with the words "equal" and "congruent" in geometry. Euclid and later educators called two figures "equal" if one can be moved to coincide with another. With the spread of set theory, the word "equal" was reserved for equality in the set-theoretic sense. But school education (in most countries) continues to use the word "equal" in Euclid's sense.