Because many drivers don't honor them. They want to charge as much as possible, and if the trip is already done, he has the advantage.
I see a lot of talk about this sort of thing in here, but it almost never has happened to me. Maybe I just look to the taxistas like someone who is not to be trifled with.
A story I have told before but it's been a while:
Many years ago I lived and worked in Peru for a a year or so. Lima is pretty cosmopolitan with a lot of English speakers but the jungle town we worked out of (Iquitos), not so much in those days. One day a new guy came into the group we worked with; he flew into Lima, got some Peruvian currency from the company agent in the airport, and immediately flew out to Iquitos. He didn't speak a word of Spanish and had no clue about the exchange rate between US$ and Peruvian soles.
He landed in Iquitos in the middle of the night and took a cab into town; the fare was 200 soles which was about US$4. When he got to the hotel the non English speaking driver told him the fare, which our hero didn't understand, so he just pulled out this wad of currency he didn't know anything about, peeled off a 1000 sole note, and gave it to the driver. The driver told him that he didn't have change (he probably really didn't) in Spanish, of course, so our guy thought he was saying it wasn't enough, so he peeled off another 1000 sole note and gave it to the driver. The driver got a little agitated, and tried to explain in slow and loud Spanish that he couldn't break a 1000 sole note.
Our hero was even more confused by this point, so he thought he would just throw more money at the problem, so he peeled off another 1000 sole note and handed it to the driver and asked him (in English, of course) if that was enough. The driver finally just shrugged, took the money, and drove off. When our hero found out that he had paid over 20 bucks for a $4 cab ride, he was not happy.