Becoming an SME is not a means to an end. Instead, people take a certain career path based on their passion for the subject, often including PhDs, spend time in academia and/or industry where they're working on one broader subject only for years, and typically see themselves in that field permanently. They don't do that thinking that they'll end up at MBB one day.
In my case, I hold various degrees in Geology & Geophysics and spent 6+ years working in upstream oil and gas projects around the globe. That could have been just enough to set me up for an SME path in MBB, had I not wanted to pursue the generalist path at McKinsey.
My point being, becoming an SME is something that takes years and isn't a viable strategy if you're thinking about it as a potential USP for entering MBB for example (I have to guess a little here where your question is coming from).
Hope this helps a bit. Best of luck!