Also a bespoke frame has to offer something different to the mass market frames. Most mass market frames are Alu not steel. Also making a steel frame is technically simplet than welding up an Alu one.
Failures in Alu frames are mostly down to the design. If they are used within the design limits then an Alu frame will ast a long long time. If the the frame is designed with limits that are easily exceeded then it won't last too long. This is the same for steel and carbon frames.