Once a business grows to certain size and beyond the shareholders don't want to hear anything about tech rewrites. They are happy the product is working, they are not alarmed by slow response times (which GitHub has plenty of on my gigabit connection that streams 4k@30fps without any lag) unless there's a big customer churn, and hosting costs, even if big-ish, to them are just the cost of doing business.
My point is, yes, you are right -- but there's a lot of conservatism involved once the business gets to a certain size. Nobody cares about improving anything from then on (which usually leads to the now-giant to start steadily losing relevance; GitHub is quite far from that but we all remember Microsoft, right?).
