We don't have the luxury of McMaster Carr here, but a quick search suggests that they have history and expertise in supplying engineering equipment which explains why they are so much better than Amazon.

It does seem like large tech companies tend to take less than stellar attempts into these sort of areas because they don't hire people with the appropriate expertise.

That said I appreciate the fact they don't for a number of reasons as it's definitely something I wouldn't want Amazon to step into and stomp the competitors in the field.