Why do they put a line though the number seven?

Answer 1

This is just to make it easier to tell a 7 with a short top bar from a 1.

In French, the number 1 has an exaggerated upstroke (line from left) rather than a single vertical line, and the crossbar is often required to tell a 1 and 7 apart.

Similarly, a line may be placed diagonally thru a zero (0) to differentiate it from a capital O : ∅.

But in Danish, the exact opposite is done, where a line indicates a capital o (Ø).