Why would they be mutually exclusive though?
Online games like '76 and ES are heavily played by people who loved the single-player games. If they release single player games, the player base for their online games will disappear for a substantial amount of time whilst they consume the new single player product. These single-player games are big: we could be talking months that people aren't playing the MMO.
This will lead to a substantial loss of income in the short-term - and also potentially long-term, because losing a large chunk of players could undermine confidence in the game, make it less interesting for other players who consequently leave, nor is there a guarantee the players who pop off to do the single player will come back to the MMO. It is not a guarantee it will damage their MMO of course: but it is a very credible
risk. To give some idea of cost, WoW is exceptional, and reckoned to make ~$1 billion a year in revenue. Something like ESO or Fallout '76 much less, but I'd guess potentially up to $100 million a year. That doesn't need to go down much to cost a great deal of potential income, especially if it does have a long-term effect across years or shortens the MMO lifespan.
There could be other issues involved - staff limitations (they need to work on one or the other - making the single player could pause some development of the MMO again with potential loss of interest), marketing confusion with two games under the same brand, etc.