The problem with leaving it to modders is that, firstly, most of them simply don't have access to good voice actors with decent equipment. I agree that voice acting isn't the be-all and end-all of romance/companion mods, but in a fully voiced game like Oblivion or Skyrim it is a problem for immersion to have text only dialogue. Secondly, there are going to be serious problems with making the rest of the NPCs in the game aware of any companions the player has. If the romance mod is fairly self contained that isn't an issue, but if you're trying to have a romance that runs alongside other quests in the game (more of the epic adventure style of romance than the hearth and home style) then it does help immersion if now and again other NPCs in the game acknowledge the presence of the companion character.
Finally, a large number of Elder Scrolls fans can't use mods, as they're playing on console. Of course, one might assume that people who play on console have no interest in roleplaying, but I think that's an unfair assumption.
A number of the companion mods in Oblivion are fully voiced. Vilja is. And not everyone has that voice over hang-up. I don't. Emma is very good at having interaction with NPCs in her mods, many of them have comments about your companion when you talk to them. Kateri's Julan Ashander is good about that too. Julan's quests (Julan is male, and only has romance options if you're female), all interact with Morrowinds main quest, and his dialog is excellent. Get him drunk, you won't regret it. He's hilarious. There's another one, Jac's Jasmine, that also has quests that interact with the main quest. Oh, and Emma and Grumpy's Constance (not a romance mod, just a companion, and my personal favorite). Each town you go into, people have different opinions about her. She likes to get you in trouble. Many of these mods interact with each other, and will make comments, and greet each other. Emma's are good about that.
I'm fully aware that some of the members of this forum don't have a computer. Right now, I play Oblivion, Fallout 3, and Fallout NV on the 360. I can only play the older games on my computer. I don't know what to make of your last sentence. I never made any assumption like that.
Peace.