Around here, there are loads of places that sell prefabricated granite countertops for both kitchens and bathrooms. I'm not sure why you're finding it so difficult.

Do they have some website to ship it to me?
What are the brand names of the countertops?

I have countertop places around me too, but they are not cheap. That $175 figure from Lowe's was $400 from a countertop specialty store.

Kitchens are not standardized sizes like vanities. Only a drop in sink will use (practically) the same size hole. Most people want undermount. There are thousands of options for the sink. Thousands for the faucets and the number of holes that they take. And thousands if not millions of individual kitchen configurations.

Some areas do have 25.5" "blanks" that fabricators then cut to size and do the sink cutouts. But, in a lot of the country, that granite is poor quality, and reputable fabricators don't deal with it. Some areas of the country have better options when it comes to quality in the blanks. And some fabricators are more amenable to doing the labor for them.

Large sized pieces of granite are not really DIYable for the fabrication or the install. Vanities are relatively small, and light, for the stone world. You need skill, specialty tools, and muscle to deal with the stone of the kitchen world.