Those CSS frameworks only enable startups to rapidly prototype an MVP without an in-house design team. It allows them to work on what they enjoy and on what makes sense considering their skillets. It would be harder if those frameworks wouldn't exist.
On the other end, when a company grows towards maturity, it makes sense to have a design team for all their products that can experiment and design specific elements that are consistent not only between themselves but also with their branding, mission and strategy.