Sorry this is inaccurate.
As the holder of an MBBS degree, I can verify you can certainly practice in the US without a DO or MD. The degree, along with the BMBS, MBChB, MC BCh, BMed, are recognized as equivalent and are not a barrier to training or working in the US.
If you have any professors educated in the UK, South Africa, Australia, Singapore, India, Pakistan, etc. they likely hold an MBBS or one of the other degrees above. In many countries, the MD is a research degree, awarded after a certain period of time in practice and completion of a research dissertation (akin to a PhD).
In regards to why the post graduate training is different in length may be related to the fact that many countries require "Foundation Years" or general medicine training after medical school before applying for specialization; the training may be less intense or lesser in quality but we are not in a position to judge that. Of note, the Medical Council of India has recently suggested that Orthopedic training encompassing the breadth of the field, needs to increase in length, perhaps to the same length required in the US, and the UK.
Finally, the US does not have sole ownership of quality medical training or care. It is naive to assume so.
