Because a game with standards is a game where standards are expected to be used, and are expected to show up as AI.
And there is a very clear reason for that, if they were to have only premium cars then half of the grid variety would be gone, and the game would suffer dearly for it.
Before the thing becomes "oh noes, is 200 MX-5s", as it happens, many standard cars help give the game some variety and leverage, even though there are as much as 200 duplicates (that I do agree should be gone from the series) people simply can't ignore much of the game content that complements the rest of the game. Many aspects of the game are based under such a thing, one of the latest seasonal as an example has included LMP1 cars, in order to fill a 12 car grid Pescarolo cars, Audi R10 TDI, and the upgraded Bentley LM car fill those grids, these cars work in par with premiums (physics and AI) so is nonsensical to have them gone in favour of eye candy, and keep the diversity at almost at 12 different models per race ratio.
If the game was released with only premiums, and an optional free standard DLC that would integrate them into every mode of the game, I wouldn't have a problem. The game then has to be designed to be completeable without ever seeing a standard. I can play the way I want, and people who love standards can play the way they want.
Forza games suffer heavily from this and sometimes the specs of the cars they choose simply struggle, making nearly every race a race against a single car model (this was the case for A class in most of the series, a car dominated the class and most of the races in career mode, it eventually became a game of beating x car for any event that had that met the a class criteria, like proper a class races, or Italian car based events, or sports cars race events, all of them different events but it would always have a F430 to beat, making racing monotonous).
In a game that is heavy influenced on numbers is really hard to halve much of the content in favour of eye candy and selected cars, old DTM cars are heavily used in 550pp and 600pp events to complement the grids, which nearly all the times have 12 to 16 different models on them, so even if people don't like them they have a necessary function within the game, I don't use them but I have fun racing against them and is always nice to have that sense of diversity in the game while racing.
Personally, if I were to choose between racing against just 3 models of a premium car, instead of 3 premium car models and 6 standards that are different, I'll just take the later because is more varied and offer more options. I do not find acceptable the duplicates (because people who want to jump into this statement well just go for it), however claiming that the game would simply not work or that would be unplayable is a different story altogether, it would alter the game visual consistency however
I do not find it bothering, though I understand why people would not be okay with it.
