blank number boards on the long hoods

I ofton wondered why don't most western Railroads such as UP, SP, SSW, C&NW, ATSF, BN, and even BNSF have lighted numbers on the end of the long hoods? I see a lot of these units from time to time and have always seen blank spots where the number boards would be on the ends of the long hoods what is the reasoning for this practice is it a money saver or cheaper to use reflecive numbers in place of lighted ones?

FROM MY EXPERIENCE, RUNNING THE UP OUT OF HOUSTON, FOR YEARS, THE MAJORITY OF THE OLDER STUFF (DASH-2, ETC) ACTUALLY DO HAVE LIGHTED NUMBER BOARDS. THE UP, AS WELL AS SP, PAINTED THEM OVER, AND JUST APPLIED DECAL NUMBERS, IN MANY CASES. THE LIGHTED NUMBER BOARDS ARE FOR THE CREWS ON OTHER TRAINS, TO IDENTIFY, BY LOCO NUMBER, PASSING, OR MEETING TRAINS. FOR THE MOST PART, YOU WON'T SEE MANY OF THOSE UP OR SP LOCOS BEING RUN LONG-HOOD FORWARD. MAYBE ON A LOCAL, OR TRANSFER, BUT NOT OVER THE ROAD. ANOTHER THING I NOTICED, WAS THE SLOW REPLACEMENT OF THOSE BOARDS, WITH WELDED IN PLUGS. THEY TOOK THE TIME TO BLANK OUT THE BOARDS, BUT LEFT THE LIGHT RECEPTACLES, IN THE HOOD. USUALLY, A COUPLE OF LITES WORKED, AND AIDED UNDERHOOD INSPECTION, OR "SEARCH MISSIONS" AT NIGHT. THIS WAS APPARENT ON RECENTLY SHOPPED ENGINES, RETURNING FROM NORTH LITTLE ROCK. DON'T KNOW IF THIS WORK WAS ALSO DONE ELSEWHERE. ECONOMICS PROBABLY DID NOT PLAY MUCH PART, AS DID LESS MAINTENANCE TIME, AND IT IS EASIER TO PAINT THEM, THAN TO MASK THEM. (ALTHOUGH, A LOT OF THE UP COLOR CHANGES, FROM YELLOW TO BLACK TAKE PLACE IN THE CENTER OF THAT REAR NUMBER BOARD) NOT BEING REQUIRED, BY THE FRA, MIGHT BE ANOTHER REASON, AS WE ALWAYS THOUGHT THE UP WAS RATHER "FRUGAL" IN EQUIPPING ITS LOCOS, WITH ACCESSORIES, AND EVEN CREATURE COMFORTS...."WE ARE UNION PACIFIC....PREPARE TO BE ASSIMILATED...."

Economics: There are about four light bulbs for rear number boards, with fleet of about seven thousand locomotives, that is about 28,000 light bulbs the U.P does not need to maintain.