Apologies if this is a dumb question but:

Why do price charts for securities like stocks and foreign exchange always show the actual price numbers instead of charting everything in terms of percent change?

The measure of how much the price changed during any period is infinitely more important than the price itself, since that's how money invested in the security is gained and lost. And it's the only way to do technical comparison of more than one security. So, why is it not the convention to chart the percent change rather than the price?