2001 Toyota Rav4 Lights Problems

TOYOTA RAV 4 2001, HEADLIGHTS FOG UP AFTER ONE YEAR. THEY ARE A SEALED UNIT. I HAVE HAD THEM REPLACED 3 TIME NOW BECAUSE OF THIS PROBLEM. THEY ARE NOT CRACKED OR DAMAGED IN ANY WAY. THE HEADLIGHTS JUST FOGG UP AFTER ABOUT 1 1/2 YEARS. THE COST OF THE HEADLIGHTS ARE $500 AND TO INSTALL THEM IT'S $180. AFTER THEY FOGG UP, THE LIGHT IS DIMMER ON THE NIGHT HEADLIGHTS AND CAN'T SEE VERY GOOD WHEN THE LIGHT IS SO DIM. I WAS TOLD BY THE TOYOTA REPAIR SHOP THAT THIS HAPPENS TO THE HEADLIGHTS ALL THE TIME. ALSO ON THE LEXUS MODEL THAT THEY ALSO MAKE. HE TOLD ME IT WAS A PROBLEM THAT TOYOTA HAS NOT ADDRESSED. HE JUST KNOW THAT WOULD HAPPEN AGAIN, BECAUSE HE REPLACES THEM ALL THE TIME. THAT IS UNEXCEPTIBLE, THE HEAD SHOULD NOT BE FOGGING UP LIKE THAT. WHY HASN'T TOYOTA MFG. DONE SOMETHING ABOUT THIS PROBLEM???? PLEASE HELP ME FIND OUT. PLEASEEEEEE. THANK YOU. *AK