Fat people are pretty common, they’re not a rare body shape or disability that should require a custom bike.

Bikes are, in general, designed to be as light as they can be for their price point. The reason behind this is that a lighter bike is less weight to move, meaning for the same effort one can potentially go farther or faster than they would be able on a heavier bike. So when a company is designing a bike, they think about the person they believe will buy it and design a bike that will support that rider.

Heavier people weigh more, obviously. Larger loads require more structural strength. Making a bike that can carry a 300lb+ person without breaking involves a redesign if you initially designed for lighter loads. Similarly, building it requires change to your manufacturing processes.

People who have health problems due to their weight, in general, do not buy as many bikes as people whose weight does not negatively impact their health. A company isn't going to go an make a big production run of an expensive product if they don't think there's a market for it, which means it becomes a custom job to get one done.

Want cheaper bikes that can handle 300lb+ riders? Do a kickstarter and see how many customers will put down dollars.