Typically they're either using the same elements despite being branded differently, or there's a competitive factor in the design that's not superficially obvio

Typically they're either using the same elements despite being branded differently, or there's a competitive factor in the design that's not superficially obvious to the consumer, or the market is so established that there's a repository of existing designs that can be slightly tweaked to create a new product affordably. Let's say you want to sell plastic brooms. With such a common product, what possible competitive edge could you get by creating your own design from scratch? If your reason for getting in the business is that you want to address a niche or trend (e.g. you want to sell hot pink brooms because it's Barbie summer of 2023), that can be accomplished with an existing design. It only makes sense to create new molds for a broom if you're a corporation like Ikea with the massive sales volume that enables vertical integration of every aspect of the product.