RJ45's are absolutely not all made in the same factory (I know you meant not just 1 but you were thinking only a few).
This is very apparent if you order a bunch of different brands and compare. There is a huge difference in the quality from cheap ones to more expensive ones.

Monoprice does just copy the designs from other manufactures in those Chinese factories. But they usually only knock-off the cheaper brands to begin with. So even if it is cheaper than a comparable product, doesn't mean the product was a good choice to begin with.

Of RJ45s, the Panduit stuff is awesome but very expensive and special tools on the Cat6 stuff. Belden makes some good stuff as well (not to be confused with Belkin).