AAA studios seem to be more and more subject to reactive panic, desperate to chase favourable YouTube clickbait absent any sort of design vision. Starfield's reception was bad and, among many other things, people bitched that the game has no rovers, so now Bethesda's adding rovers. Doesn't matter whether the gameplay structure can support them in any meaningful way, what matters is the ensuing viral marketing that "Bethesda listened!" and "Starfield is saved!" The same thing happened to Cyberpunk 2077 - bad reception, people bitched about not having multiple player homes (again among many other things), so CDPR added purchasable apartments despite being completely pointless in the game's loops. People bitched about the Metro fast travel, some obsessive modders even made something themselves, so CDPR added one too despite being - guess what? - completely and utterly pointless.
This is a misdiagnosis, you say they are being reactive when the issue is that they don't have any sort of coherent design or vision in the first place and can't deliver on quality and fun. You also can't blame marketing for this, they're just putting out there what the team is saying they're making and are planning on.
Cyberpunk 2077 was a vaporware mess that put game devs through the meatgrinder as the leadership failed on all levels. When they were marketing car customization and tuning, several apartments and a train system in the game's PR they were writing IOU:s, making promises they failed to deliver on. You're right that these alone without the vision of the game they were a part of that was cut down into something completely different don't do much for the game. What they sold the game as, and in many ways seemed intent on making the game into, was something with GTA elements. A focus on driving, the vehicles themselves, the open world dynamic shootouts, a Cyberpunk world-space fully realized. In that context the trains made a lot of sense, the GTA series had them back in GTA2 (and in the first game but the second was more futuristic) and they contributed an additional way of navigation, allowing you to skip long drives and just go above or underground directly to a location, and if you were chased by law enforcement or anyone else it was an interesting place for a pursuit, the train tracks electrifying anyone who walked over them just before the train arrived. They make sense for the same reason they make sense in the real world if you're going for a world simulation.
In the end Cyberpunk 2077 wasn't GTA though, the wanted system was entirely broken, vehicular sections were few and entirely scripted, and the GTA convention of trains, being able to buy apartments and properties, and the car customization which were all cut didn't really have a place in the game anymore after the failure to deliver on that larger design promise reducing them into pointless fluff, but at least it was additional content. The game we got was Deus Ex: Human Revolution done in the style of Ubisoft, highly cinematic FPS segments with a lot of trashy busywork entirely lacking the dynamism of GTA.
No Man's Sky wasn't reactive either, they just completed the things they weren't able to before Sony's deadline hit them, but unlike Cyberpunk the vision never changed to such a degree as to invalidate this cut content they had been marketing.
Starfield never did market rovers or vehicles, so in this case they are reacting to feedback, but to a failure of their own game design and not "bitching". They actually thought people would have a great time slowly walking across large empty sub-Daggerfall procedurally generated squares and that this would be just as fun as the totally curated moment to moment gameplay of Skyrim, Oblivion and Morrowind. It's even weirder since you had horses in the later TES games but in the far future of Aboriginal soul-stares there are no ground based vehicles for transportation. Rovers are actually fixing a problem the game has, the issue is that none of the content is worthwhile and playing Starfield is a terrible experience in the first place, marred with issues so fundamental they could never fix it without remaking the game from scratch. Getting to content will be quicker, but because that content is bad it doesn't matter.
It's not the fault of marketing, it's early design failures and leadership failures to maintain a consistent vision and ensuring that the various part of the games mesh and make sense.
