Unlike EA, Epic does not have a diverse portfolio of games on different devices. EA can absorb a flop while Epic is clearly pushing people to install their PC launcher through their free Battle Royale to get potential customers to pay for their paid content (Fortnite, in-game purchases, other games, etc.)

For SimCity, most of the sales were already done, so EA could have chosen to do the bare minimum because they would only see small returns for those fix. Whereas for Epic, the business model is different since it's F2P, so they'll need the player to keep coming back so they keep buying stuff.

Because Free For All games are really popular right now, and they want their engine (Unreal) to be a competitive choice for these and other massively multiplayer games.

Unreal is already an extremely competitive choice for multiplayer games (read: the best choice). They are putting resources to solving these problems because they are making millions of dollars from "free" players.