You may use sympref function with 'AbbreviateOutput' parameter set to false to avoid the creation of new variables
sympref('AbbreviateOutput',false);
syms K EI l1
K1 = [12*EI/l1^3, 6*EI/l1^2, -12*EI/l1^3, 6*EI/l1^2;
6*EI/l1^2, 4*EI/l1, -6*EI/l1^2, 2*EI/l1;
-12*EI/l1^3, -6*EI/l1^2, 12*EI/l1^3, -6*EI/l1^2;
6*EI/l1^2, 2*EI/l1, -6*EI/l1^2, 4*EI/l1;]
Refer this documentation for more information
