One of the reasons people like SPA so much is componentization.

Here's my header component, and all its scoped CSS, and here are the 5 subcomponents that make it up, and all their individual scoped CSS.

Page transitions are 0.001% of the desire to go the SPA route.

Fun fact: custom elements exist. They're just as hard to work with as React components (i.e. complete nonsense until you stop fighting them and do it right). Except they don't violate the browser model and don't require you to be married to a specific framework.