The question asks why some martial arts exist when others are more complete.
The answer is: The rule set. It always goes back to the rules. Why something looks the way it does depends on the rule set. Change the rules even a little, and the way a martial art looks can change quite a lot.
Do the rules require a gi? Big difference between that style and one that doesn't. Are chokes allowed? Are strikes allowed? What about throws and sweeps? What about small circle joint manipulation? Etc.
Even in martial arts which have no competitive rules and never spar, there are rules. These rules just reinforce their conceptual framework. For example, in many forms of kung-fu, wrestling on the ground is considered either wrong or a form of fighting that's lower class and therefore not worth training. In classical Japanese Jujitsu, you won't see a lot of high kicks or snapping punches. The reason is because it was made for use on the battlefield where people were expected to wear armor which made those techniques impractical or impossible to do without risk.
Yes, maybe in the larger sense, two different martial arts may have the same goal, which in this case might be to pin someone to the ground for X amount of seconds or to submit them. But the rules dictate how you can get to those outcomes.
You don't see a lot of different looking MMA fighters, for example. Oh, they all have their unique style, but the variation in the way they actually move and look is very small when compared against people from a traditional martial art such as Aikido or Wing Chun. It's because the rules of MMA dictate everything. If an MMA stylist tried to suddenly switch to a traditional martial art style while in an MMA style fight, they'd lose. So they'd learn really quickly not to do that. That's why they all look very similar. They are moving in a way that's optimal for the rules of MMA.
Now, the next question you'll want to ask is whether the rules should or shouldn't be the way they are. You might ask why one martial art allows chokes and the other doesn't, for example. That sort of question, highly focused on one aspect of a particular martial art's rule set, can lead you to a much greater understanding of that style.
Judo, for example, never allowed striking, even though it's in some of their kata (yes, they have kata!). If you investigate this, you'll find that it goes all the way back to its founder, Jiguro Kano, who when asked about this said that he did not know how to incorporate striking into randori in a safe manner. And one of the main concepts of Judo is that it exists primarily as a way of physical and moral improvement. Its purpose was not to make a well-rounded fighter.
And so the higher answer to the main question is not about the rules. It's about the core philosophy behind the art. The rules are merely the manifestation of that philosophy. Asking why the rules are the way they are will lead you to understanding why that martial art exists.
Hope that helps.