I actually assume its because many of the cards do not operate against a global rules set.
So one can probably assume that Shock, Lightning bolt, Lightning strike all work the same and just apply a game rule against the system while played because they are simple. Likewise all vanilla creatures.
When you get into cards that have confusing or frankly illogical rulings based on timing, global effect ordering, layers, etc, they likely have to code the individual card to function against it's own rules set to make sure it interacts with all the other cards in the game that it applies to. Once you do this for one card, it has this cascading effect that when other cards come out that interact with that card in non global ways you have to develop unique logic not only for that card, but then for other subsequent cards that already exist, potentially not only editing that cards logic but the older cards as well to have said unique interaction.
That being said, it's not just magic that suffers in this regard. Games like Hearthstone, which are digital only, also suffer from this issue almost every single release. There are so many cards in that game that function in such a unique way that they literally cannot avoid coding specific logic into specific cards just to get the interactions to function.
The one that always fascinated me in that game was the interaction between Flare and Counterspell. Counterspell is literally a thing you put into play and triggers when an opponent plays a spell, countering that spell. Flare is a spell that wipes out all secrets, of which counterspell is. Which wins?
Now in Magic, we have the stack and we can logically work that interaction out based on when in a turn which was played, etc. In Hearthstone the interaction has see sawed back and forth as sets have been released based not only on the game rules but the intention of the developers to ensure the card works as they intended, not specifically how player base or even the game rules interpret. There have been times where similar card interactions have come out that mirrored this one and literally worked differently just because of how the global game engine interprets them.