macOS actually does have a built-in way to merge PDFs, through Preview's thumbnail sidebar — but it's a manual drag-and-drop process that gets awkward past three or four files, and it's easy to accidentally drop a page into the wrong spot.
You can open one PDF in Preview, show the sidebar, then drag pages or entire files from Finder into that sidebar to append them. It works, but there's no reorder-then-commit step — you're editing the live document as you go, and undoing a mis-drop mid-merge is finicky with several files open.
Because you set the full order before merging, there's no risk of half-editing a document and losing track of where you are.
Ready to try it yourself?
Merge PDFs on your Mac →Not bad, just manual — it works fine for two files but gets harder to manage the more files you add at once.
Once the page has loaded, yes — the merge itself doesn't require an internet connection.