Because it makes more sense than NaN which would be the other possible solution.
Number + Text = Text, and the Number is interpreted as the character that compose it in a human reading rather than based on the char matching ASCI or some other table.
The attempt to convert text into numbers to make the operation would give NaN most of the time (Not a Number)... Or should we get the asci value of "1" and get 1+"1" = 50?
I think 1+"1"=11 is a good default, as you see clearly what happend and how to correct it if it is not the result expected…