That way doesn't scale well. Things become very messy and unmaintainable really quickly. This is how people code in the past, and there are good reasons why people moved on.
Code becomes unmaintainable when it was organized poorly and grows or if discipline was not maintained during growth.
React projects are just as susceptible to that, and "modern" web development is quite frankly a larger maintainability headache due to the dependency hell they bring.
