They don't, because they don't need to. The point of the yellow dots is to prevent the printing of counterfeit currency, which itself tends to require the use of more ink colors than only black. It's probably possible to refill a "black" ink/toner cartridge with the exact shade of green (or whatever) to replicate the color of a currency note, but if it was easy to do then there'd probably be a lot more counterfeit bills floating around.

The ink/toner is also half the battle. The other half is the paper, since obviously the US Treasury doesn't use ordinary printer paper to print $20 bills. The usual trick is to take a $1 or $5 bill, bleach it (or otherwise remove the existing printing), and print a $20 bill design onto it - but that's easier said than done, due to both the ink/toner color issue mentioned above and due to the difficulty of getting the donor bill exactly aligned (and doing so again, in the exact same way, for the other side of the bill).