>I wonder why companies don’t do this anymore. Is it something to do with the monopoly AT&T held, is it related to corporate tax structures, is it related to how easy it is to find PhD graduates who studied similar topics of interest, or is it something else entirely?

Companies do a lot less to retain employees - fewer pensions, fewer accrued benefits and perks - so it's much less attractive to train when you are assuming attrition instead.