Q. What is the reason behind spelling out numbers below 10? I feel that numerals increase the clarity and reduce text length.
A. You’re right. Numerals are shorter than words, and they are arguably easier to read. Plus, an all-numeral style would make an editor’s job easier. But the digits in a number like 7 imply a precision that’s usually reserved for the sciences and other technical contexts. Even in the sciences, numbers in the form of ordinals are often spelled out below 10th, on the principle that an ordinal refers to a ranking rather than to a precise quantity.
On the other hand, numerals are customary in certain contexts even in the most literary of prose. In Chicago and most other styles, for example, you’d use digits to refer to page 3 or page 115, a bulb of 40 watts (or 450 lumens), and a 3 percent raise issued on February 1. But aside from these and (pun alert) a number of similar exceptions, words are still Chicago style below 101 in nontechnical settings and below 10 in journalism and technical (but not purely scientific) contexts (see CMOS 9.2 and 9.3).
You can blame the persistence of spelled-out smaller numbers outside the sciences on a combination of tradition and reader expectations, two (not 2) factors that tend to reinforce each other.