Totally different caching techniques. MPAs have a built in mechanism called back/forward cache. It works automatically on most sites, though some invalidate the cache. https://web.dev/articles/bfcache
SPAs need to hold onto the data themselves, and correctly transition it between pages/components. Poorly-built ones will simply refresh it every time, or discard it if the data is too large.
Both mechanisms can allow for immediate page loads if they've been correctly implemented. They just require a different approach.
And frankly, most big sites in my experience break bfcache. For instance: YouTube, Gmail, NYT, steampowered.com, CNN. All have broken bfcache in Chrome.
SPAs need to hold onto the data themselves, and correctly transition it between pages/components. Poorly-built ones will simply refresh it every time, or discard it if the data is too large.
Both mechanisms can allow for immediate page loads if they've been correctly implemented. They just require a different approach.