Because anything we might start now would be overtaken before it is one tenth there by anything we might start five hundred years from now. And if we won't be a

Because anything we might start now would be overtaken before it is one tenth there by anything we might start five hundred years from now. And if we won't be able to start anything faster by then, there would be no technological civilization left to receive the results. So it would be pointless, space is just too big. Once we can reliably reach 1/10 c we might think about sending a probe to Alpha Centauri, although even that would have more symbolic value as a technological demonstration than scientific value. You would have to wait more than half a century for a science mission of at most a few hours, because decellerating would require so much time and fuel to make the whole mission infeasible. A mission to Gliese 581 is a distant fourth or fifth project after that, and sending a planetary probe like the one we send to mars is even more distant. (For reference, it took several years of swing by maneuvers to get a probe out of the orbital plane of the planets into an orbit that allows to examine the poles of the sun, and that takes far less energy than even reaching 1/100 c.)