Because then it would be usable by 100% of the population. This way you can make it useless for 10% of them or so.

why not just put the Ohms right on the resistor?

Back in the olden days it was easier to paint on the lines.

They do print on the numbers for some modern surface mount resistors. They use a 3 number system because there isn't much room. The first 2 numbers are..... well.... the first 2 numbers of the resistor's value. The 3rd number is the amount of zeros you need to put after those numbers.

So a resistor that is 2200 ohms, would have "222" written on it. Twenty-two with 2 zeroes.

why not just put the Ohms right on the resistor?

The style of resistor that this color code was intended for can be oriented in any direction when put onto a circuit board. If the writing were only on one part of the component, the resistor could be soldered in place where the resistance value is not visible which would prevent easy identification of the value. Surface mount resistors do have a numeric code on them, but being small rectangles, they need to be oriented with the code facing away from the board. These surface mount resistors are often machine placed so the pick and place machine makes sure they are oriented correctly. Extremely small surface mount resistors do not have room for any code so they are not labeled or colored. It's a bit of a mess really.