The problem with mod-ability is that it is a feature you want in single player but it is a feature that can ruin multiplayer. Ever see a FPS multi-player in the late 90's early 2000s? It is riddled with cheats and tweaks all designed to give the player with them an unfair advantage over players without them. This is something you DO NOT want for a multiplayer environment especially competitive vs co-op multiplayer. But even co-op multiplayer can be ruined as you find that it is the mod player and his side kicks vs a team working together.
In single player environments mod-ability can extend the life of a title and also encourage DLC sales as mods made with the requirements of said mod can drive sales as many mods will use assets from the dlc. This is a win win for both the consumer and the industry as the consumer gets more value from the DLC and the industry sells the dlc to consumers that on the face wouldn't buy the dlc as the 'official' content didn't appeal to them. Also players have this idea that it is simple to create modding tools as the developers already have them but developers have 'industrial' secrets just like any other business and it takes effort to 'sanitize' your developer tools to make mod tools. And it isn't as simple as some player imagine in their own little worlds. We can see this with CDPR which is a very friendly company towards mods creating their tools for witcher 3 that are highly restrictive. Why? CDPR has always been rather open towards modders and in fact encouraging them, and they clearly stated they want to make witcher 3 a moddable game. So why the seemingly about face? Some gamers will claim the company lied because in their world developers always intend to deceive it never is a case of problems in development get in teh way of lofty goals. Nope it is always the silent film era villain intending to lie to consumer to steal their money. It is likely that CDPR ran into difficulties trying to separate their proprietary secrets embedded in their dev tools to create their mod tools. They may resolve this in the future but they may not. Again just an example how making a game moddable isn't as simple as just doing it.
The simple answer is to just say make two engines but engines are expensive and third-party engines are almost always made for the shooter market not the RPG market. As developers strive in the multiplayer market to make engines less mod-able you are seeing even third party engines adding 'features' to their engine that make modding more difficult. So even the newer engines that can be adapted for RPGs are getting harder to mod. Also once people learn how mods work for one game using X engine they can apply said knowledge to mod different games using the same engine. Its not exactly a 1 to 1 ratio here but the knowledge can be used to open other titles. So giving mod tools for one game using a given engine can hurt the 'integrity' of other games using the same engine. So developers especially third party engine developers don't want to hurt their own market. They want to show multiplayer developers that their product can be secured so player wont have to deal with cheating as that can and does ruin a multiplayer environment.
Games are incredibly expensive to make and EA has made a business decision to have all their developers use the same engine. Is it the 'right' choice? maybe, maybe not only time will tell but it isn't a stupid choice or a mean spirited choice it is simply trying to make the most use out of the assets the company owns. While I personally prefer RPGs hands down over shooters the market shows that most AAA gamers prefer shooters over RPGs, so making a business decision that favours shooters over RPGs isn't folly. If the single engine for all developers works for EA and by works means saves them money and doesn't cause undue headaches in development trying to get the frostbite engine to work with most EA titles, then I think you will see other larger companies who own multiple studios doing the same thing which is bad news for modding. If it turns out that it isn't any cheaper in the long run or not significantly cheaper to justify the development headaches then EA will drop this idea and we MAY see modding getting easier for EA titles.