I don't mean to imply that the MD's kids aren't qualified for the roles they would/will receive, just that those qualifications are a far cry from the qualifications needed for the quant space and isn't hard to do
in comparison. The men and women I know who go into the top consulting/IB/PE jobs are usually some of the the smartest and best in their class and they worked very hard for it. However, you can come from almost any college in the world with some business degree and be up to scratch with most of your job in a year. You might lack some fancy excel stuff, but there is a reason that quant shops will hire Ph.D's with no finance experience, it isn't hard to pick up (and the Ph.D in math is much harder to teach someone on the job).
The market for men and women who are willing to sit at a desk 100 hours a week for $200k is quite saturated; the difference lies in the work done during the 100 hours, the breath quant work can't be learnt in a year on the job by a random guy who is willing to work long hours. If you show up and you don't know your stuff you are useless, from what I see there is very little busy work to give you while you figure it out. The number of internal staff's children willing (and able) to get to that level in math/stats/coding ect. is much, much smaller.
But, more importantly, the difference in worth between a guy who went to an OK undergrad, sorta learned real analysis and took regression/time series vs. the man/woman who was an IMO gold medalist, has a Ph.D in Probability and Stochastic Calculus, and codes like a second language is enormous.
Meanwhile, the difference between someone who knows excel really really well and has all the accounting ratios memorized and the CEO's descendent who is about 20-40 minutes behind the other guy all day isn't quite as big. It is still an issue, sure, but it is a different kind of issue entirely. At the end of the day, banking is a time suck; but if you get in the door, you're willing to work whatever hours they tell you, and you are somewhat competent, I haven't heard of any reason you'll be out of place. But among all the top-tier men and women I know who went standard banking... I really can't tell the difference in any of their educations- there is no differentiating factor there.
The bankers I know are intelligent, incredibly driven hard workers. They typically earned their spot, I respect the effort, talent, and drive it took to do it. However, it doesn't take anywhere near as much effort to become qualified for IB work. When it comes down to hiring a relative of the man/woman who will give you your bonus (or not) vs. an overseas kid who needs sponsorship, there just isn't much of anything that overseas kid can offer that the saturated job market here in the US doesn't have in droves. But in the quant space, for the reasons listed above, overseas applicants can stand out because they are often more competent than the US kids - and when it happens it is very noticeable.
I don't disagree, but that isn't the point.
@MRoss is one to answer this part; but he said above that they don't. I'm just speculating as to why they wouldn't need to sponsor, I don't have the experience to tell you whether or not they really do.