What prevents platelets from clotting regular blood vessels?
Platelets activate when the exposed collagen (which is normally present below the endothelium) at the wound site binds to the platelets, forming a plug. Other factors are also secreted which aid in stronger binding. A signaling cascade is triggered which then makes platelets release proteins called integrins, which help for tighter binding of platelets at the site. Activated platelets also release many stored compounds from their granules which initiate further signaling cascades to form a clot. 
In a healthy tissue, there is no exposed collagen. Ergo, No clotting.