I would say just the opposite is true. The real world is packed full of things that exist "just because". Things that exist that are irrelevant to the PCs and the adventure are what give a world it's depth - the illusion that it exists independently. The Lord of the Rings (and the Hobbit, which mentions the fall of Gondolin) are chock full of references to things that are of no relevance and are not explained. That what makes Middle Earth feel so deep, when the fantasy worlds of other authors feel so shallow.
I'm not 100% up on my Tolkien lore, but I'd argue that not a single thing in Middle-Earth existed "just because". I have no idea why the Balrog was in Moria. Reading the trilogy gives me no insight into this. But, I do know that in other works Tolkien actually explained the history of that Balrog and explained exactly why it was where it was. The Fall of Gondolin is never explained to us in the Hobbit, but it is still an intentional part of the world and is mentioned because of the ripple effects it had.
Everything in Middle-Earth was placed with an intent.
To put it another way, you don't just throw an empty pyramid into the middle of a plain, with no purpose, story or hook. Why is it there? I don't know, just because. Somethings just exist in the world for no reason.
It is a massive pyramid, someone had to have built it, it had to have a purpose, it can't exist "just because" it has to have a reason. And if it truly doesn't, then why did you even bother telling us about it. It might as well be a tree, or a stone, or a cloud. It add nothing.
Because the author (Ed Greenwood?) Chose to make her evil. It matched his perception of the sea. All Art is subjective.
But FR has a plethora of gods - if you want to add another sea god with a different temperament there is no reason not to. In certain editions the rule for the FR was "all casters of divine spells must choose a god" but also "if you can't find one you like make one up".
Woo. Wonderful.
And a complete cop-out. Weren't you just complementing Tolkien for his believable worlds? Do you think Sauron was evil "just because, and if you don't like that you can just make a good version of him."?
We don't care about authorial intent, not in this case, because the rules have been set out for us, and we are trying to make sense of them.
Umberlee is a Goddess of the Sea
She exists because people worship her
She is evil and cruel
People only worship her out of fear
There does exist a good god of the sea
If everyone worshipped him instead, Umberlee would die and the seas would be safe.
So why hasn't that happened? Why did people start worshipping Umberlee? We've been given the rules and the characters, but the story is nonsensical.
Except real world pantheons and religions did have evil deities in their own pantheons and belief systems. They would not necessarily worship them, but they were still often part of the belief system. When you deal with absolute statements like this about real world beliefs operating in a certain way, you are bound to be contradicted. As I said - which you some how ignored - Yam was an evil Ugaritic deity who was regarded as evil by the Ugaritic people.
Alright, but here (to tie this back to the OP) is the question. How many of those religions with explicitly evil Deities also had powerful demons that were threats to the gods that were not tied to those gods.
For an example, from what I know of Norse mythology none of the Aesir were evil (barring some twisting of Loki at the end), none of the Vanir were evil.
The enemy were the Jotuns and the Fire Giants. And they weren't gods, but they were as powerful as the gods.
Or take Hindu Mythology (which I have an admittedly limited knowledge of) there are no evil dieties worshipped, but their are evil spirits and demons that the gods fight.
DnD though has both. We have Evil Gods, we have Demons, and we have the Devil. All three are existing simultaneously as foes of the gods of good. But do we need them to? Why have redundant aspects.
Especially when you start looking back at things like, who you fought if you were opposed to say Bane.
You would fight clerics and cultists of Bane, and you would fight Devils. Because Bane didn't have angels, he had Devils serving him... but all Devils follow Asmodeus, so why would Bane have Devils? Because they were the Lawful Evil outsiders. Or Lolth having demons that follow her.
If they are using Demons and Devils anyways... why are they not Demon Princes and Archdevils? It would fit perfectly. Or, remove the Demon Princes and Archdevils and have all the antagonists be evil gods, and the followers of evil gods are demons and devils. That also works.
But both is just messy for no reason.