While planet exploration certainly would be a feature i'd love to see, i think there are some things i can think of why it would be, compared to NMS and Elite, far more difficult to implement:
1) CPU-power
When you play NMS or Elite your pc does not have to simulate the entire rest of the universe while you're hopping around on some planet or even when flying through space.
In both Elite and NMS when you travel from system to system with your jumpdrive the entire sector you're leaving gets wiped from your computers memory and only the system you're in gets loaded as a small level. When you land on a planet, almost everything in space vanishes and only your planet is kept as a new "sublevel". These two games essentially only show and simulate a small bubble around the player, they don't have a need to calculate thousands of stations and ships if they aren't in the same "level" as you.
X games on the other hand do have to calculate everything that's going on even if you are on the other side of the universe. While that is already quite taxing on the CPU while in space, imagine having to do that while also rendering nice-looking planet environments and calculating possibly thousands of on-surface vehicles per planet.
2) Ai
Fighting on the ground or with atmosphere-only vehicles would require Adding at least 2 new Ai systems : one for ground combat, one for atmospheric flight.
Ais for all ships in the universe can be very CPU-intensive, so i refer to my point 1), however i also like to add that the space flight AI in X games already isn't that smart (my guess is because there are so many Ai they have to share CPU ressources). Imagine how ships like to bump into eachother in space, now imagine the X ai flying moves in a gravity-affected combat aircraft over a fatal planet surface....
i'd rather have them improve the space flight Ai before them adding two new types of Ai for completely different environments.
3) Planet creation
While in Elite you can on "planets" as far as i know you can't really land on all planets, but rather "only" on non-atmospheric objects, and not all of them, proably because generating and loading believable planets with vegetation, animals, etc is far more difficult. Many planets in the X-universe however have life and atmosphere. Lets say Egosoft copied Elite and implemented planetary landings for non-atmospheric planets, what would be the in-game logical reason why you would be able to land on some planets but only those without plants, cities or any form of life?
Ok, lets take NMSs planetary generation then, it can generate planets with plant life and animals, so we can land anywhere. OK, it can't generate cities properly, but lets say that's not important for now.
While i absolutely enjoy playing NMS even to this day, i must say that after a while many planets feel very generic and same-y. Procedually generated content is bound to feel generic after a while, even while watching Star Citizens city generation technology you can see that many buildings and objects are repeated in a pattern. Genericness and repeteition is the biggest flaw with current planetary procedural generation and i have yet to see a game with procedurally generated planets that truly makes every planet feel unique and believable.
The only alternative would be creating the planets by hand, that that'd be a stupidly huge amount of work, especially if you want to uniquely model every planet in the X universe.
Again, i'd too love to have planetary exploration, but until the flaws of the current-gen planetary generation algorithms (genericness, believability of plant/animal life and cities, on-planet entity Ai and CPU-consumption) are improved and Egosoft somehow gets additional teams so they don't lose ressources intended for developing the space-part of the game i see little benefit in adding them to the X-series.