They have "type" certifications.
A vehicle is certified by those regulating authorities, and then variants of that model are grand-fathered in, or ammended to the certification.
A car with 5 different trim models, including 3 or 4 different engine choices... the model is type-certified, and then the differences are ammended to the main certification.
US Federalizing... Each of those various engines has to be almost fully federalized in and of itself, as if it were a completely independent model. Having lots of variants raises the costs of federalization dramatically.
So the models we do get have fewer niche configurations, fewer distinct choices, and all tend to be re-treads of the same mass-market appliances that the companies know will sell, and recoup all the federalization costs, in addition to the production costs.
And many of the EU countries, if it meeds German TUV, or British MOT, or even the Japanese or Australian equivalents, will grandfather that in to be acceptable in their countries also. There is no cross-compatibility with US-federalization, coming into the US, even though most of the criteria for certification in most cases of the developed automotive market countries are reasonably equivalent, and probably could be negotiated to be cross-compatible, and thus much less expensive for auto makers, and thus automobile buyers.
Less expensive would cut costs, and allow manufacturers to make profits on smaller numbers of units sold, and niche models, such as performance models, would be easier to justify adding to the lineup.