If we compare the traffic of Github vs Bitcoin, Github is likely doing 1,000+ writes per second and Bitcoin is doing what, 5-7 maybe higher with specialized stuff?

Github is nowhere near the world's "central and only" service for Git, so what am I missing to not laugh about?

The downside of a global distributed database (no matter what) is the speed of light, if you need ordering in any transaction you are in trouble, and no classic service requires that for all transactions in its scope, we figured out partitions, row locks, and shards a long time ago.