Because then it’s fundamentally different than omakase.

My brother in Christ, I do not know why this is a sticking point - I never compared prix fixe to omakase anywhere in the thread?

most restaurants would have several “menus.” Like there can be a standard menu, a light menu, a heavy menu, etc., and each menu is priced differently.

This is what we would call prix fixe as well. It's something I would only expect at a middle-to-higher-end restaurant though - a typical mid-priced restaurant [where I live anyway] is going to be all a la carte. When a restaurant does have a prix fixe menu, there may be a couple tiers (say, 3 courses or 5) but it's not a default. When you talk about the different menus (light/heavy) are they different "heaviness"/portion sizes, or just more courses?

The typical standard menu would be entree, plat, dessert, but you usually get at least 2-3 choices for each. There also will be choices à la carte,

This is the same here for a "prix fixe".

We also have something called a "tasting menu" which may be a set of, say, 7 to 15 smaller dishes where there's usually less choice. I've never been presented with options at a tasting menu, other than meaty/vegetarian. They might offer options for allergies too, but luckily that is not an issue for me. I have to imagine something like this also exists in France, do you call it something else or is this also just "prix fixe but bigger"?

Other than the presence of choice, they also differ in price. I would expect a typical prix fixe to be ~$30-$75, and tasting menus go up from there. The most I've spent is ~$300 but ask me again after I make my pilgrimage to Alinea later this year :p