Why Can't We Directly Give D.C to Alternator Rotor?

Hi,

Normally in A.C generators we have brushless D.C excitation systems in which first of all d.c is given through external source to excitor stator,Then a.c is generated in exciter rotor which is rectified by rectifiers to d.c and then it is given to main alternator Rotor from which 3-phase a.c is produced which is taken out from stator,My question is why can't the d.c be directly given to main alternator rotor without any need of exciter at all,Thanks.

Sohan