The "default" is a 32 bit int, large enough for almost 4.3 billion records. That is enough for the vast majority of tables, and going to a 64 bit int has some performance implications, which at GitHubs scale is absolutely something they need to take into consideration, just as they should have with the table size.

I've set up systems where I chose 32bit int because I could've never imagined to need more than 4 billion values. AT some points I did (related to deletes) but changing the datatype when you have >1bn records can be next to impossible for 24/7 operations, especially for primary keys.