because metal expands and contracts with heat.
there is a huge temperature difference between the feed pipe from the manifold to the wastegate and the dump tube from the wastegate. one will grow considerably more than the other. plus you then solid mount it to the downpipe which again will be expanding and contracting at a different rate.
then couple the fact that stainless gets brittle with heat, and factor in the shock movement from the engine.
when i've build turbocharger kits for the 3sgte's i specalise in. the dump tube has a flexy unit on it, but also so does the downpipe after the 2 join to the rest of the exhaust system. This is ontop of the stock flexy that is present futher down the line. over 100 kits produced never a cracked part or downpipe
Tim