It’s in the name: an MPA requires a page roundtrip for every navigation, while SPA requires a single page roundtrip. Any subsequent requests are part of the application itself and can be handled without disrupting UX.
An SPA still requires roundtrips, just not to load the DOM root. Most of them do more than one roundtrip per navigation, making them worse than MPA. They also break loading screens.
An MPA can make as many queries/service requests as it needs to to render the result. Most— the vast majority of SPAs I’ve ever written, maintained, or used— do that from the client, making many requests to render the new route.
The UX for an SPA can indeed be excellent, but the average SPA is worse than the average MPA, in my opinion. It’s just much easier to break things in many subtle ways in an SPA.
Is it really your experience that SPA data fetches and refreshes are "handled without disrupting UX"? I think it's the opposite. When a page is rendered on the server it's much less likely to move around under me, forcing accidental misclicks or shifting what I'm reading off-screen at random.