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I think the bloat comes from a few places:

Developer productivity: Making complex pages and reusing components across pages is much easier with a library like React than with more vanilla approaches. For a lot of companies with large numbers of engineers, making sure that engineers can be productive without intimate knowledge of HTML/CSS ends up taking priority over things like performance and accessibility.

Branding/customization: The built-in HTML controls are difficult/impossible to style or customize. A lot of UI designers will design some fancy looking select dropdown, not appreciating the fact that they're forcing developers to reinvent the wheel in order to implement their design. Alternatively, there are cases where the UX for built-in controls is lacking enough that you're somewhat forced to implement a replacement (e.g. <select multiple>)