they undergo high tensile forces, so have to have strength (have you ever seen a really big person jump?). On the other hand, they pass the force created by the muscle to the acual joint doing the work, if they were stretchy they would start absorbing this force instead of transmitting it.
Miss sixty, you wouldn't happen to have a biology homework assignment, would you? Or shall we assume that you have a newly acquired fascination with ligaments and tendons?