becuase then the 3 lines woul dnot connect
if you draw any 1 line of legnth w
then lets say you draw a line of legnth w/2 and (2/w)-1
then you would have 2 lines that are too short to connect
try it yourself
basically lets say we have sides legnth 3,6,10
10-6 is more than 3
if you draw a line that is 10 legnth and try to draw 1 line at each end, one which is 3 legnth and the other which is 6 legnth, you cannnot connect the 3 and 6 line becuase they are 1 unit too short, if they were 1 unit longer, then the triangle would be a line