Why do teachers have to spend their own money?

One question I've had for years is why so many teachers have to spend their own money in order to have a quality program.  In California at least, most teachers have very little, if any funds to spend on extras to make a program interesting, and some even have to buy their own art and science supplies.  Given that some districts do not even have health insurance anymore, and I know teachers that therefore do not have health insurance and live from paycheck to paycheck, this is a crime.  Many people do not know that teachers do this, other than teachers friends and family.