This is the big one.
Github’s ability and dominance in the social space means they have the most active actions ecosystems.
They made actions easier to import share and built marketplaces and now gobs of developers build value into it just for clout (and because they enjoy software of course).
If you think about it carefully, it’s actually a security nightmare. You have no idea what a random Github action you “uses:”d is doing, but in practice 90%+ of the time it doesn’t matter, you get good functionality and save time for free.