As others have said. Most FSN characters (other than Saber and Gil) are lower rarity specifically so that players have a chance at getting them. It's a money thing.
Again, as others have said. The writers have actually acknowledged that this was a mistake - Romulus, being both a demigod and a founder of one of the Foundations of Humanity, really should have been a lot more powerful. As the character points out, he is conceptually Rome. And since so much of modern civilization is built on Rome, then conceptually, he is one of the foundational elements of modern civilization. He should, in theory, basically be a god of the modern world.
Jokes are the deepest lore. This is a fact. One-off jokes made by characters during events, or gag illustrations by Riyo, inevitably turn into actual plot points or Servants.
So we shouldn't be surprised that the opposite is also true - lore becomes a running joke.
I know we have a lot of Sakurafaces, but that's canon - BB was based on an AI based on Sakura, and the BB Five were based on BB. Kama and Parvati are odd choices, but granted. But do we have that many Rinfaces? We have Ishtar, and Eresh, and Rider Ishtar, and... yeah, come to think of it, we're due some more, aren't we?
Okay, point retracted.