Oh, the macro is that these companies are oligopolies. About 15–20 years ago one of them realized that poaching entire teams from the others to enter new LOBs was cheaper than competing. So headcount grew.
Outside of strategic hires it doesn’t really matter who they pick up. E.g. LinkedIn isn’t going to go out of biz if they don’t find productive places for their army of level 3.5 software engineers or whatever. LinkedIn doesn’t have any competition.