Trees and mailboxes are actually fairly sturdy. These were probably fragmentation bombs built of a kitchen appliance and filled with ball bearings and nails designed to spread shrapnel over a wide area against soft targets (people
) instead of force to nearby hard objects.
It's not an explosion like in the movies which are rigged to send big objects flying.
Pressure cookers also don't contain enough pressure to cause that much concussive force. A pressure cooker is usually rated to cook only at like 15 PSI (and go a bit higher for safety reasons) while a standard pipe bomb is probably over 1000 PSI.